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# Chapter 2: Yodl User Guide

This section describes the `yodl` program from the point of a meta-user, one who is interested in how macro files work, or one who wants to write a new converter. If you're just interested in using Yodl with the pre-existing converters and macro files, skip this chapter and continue with the macro package description (chapter 4).

The `Yodl` program the main converter of the Yodl package. The basic usage of the `yodl` program, `yodl`'s built-in macros, and the syntax of the Yodl language is described here.

## 2.1: Using the yodl program

`Yodl` reads one or more input files, interprets the commands therein, and writes one output file. The program is started as:
yodl options inputfile [inputfile...]
In this specification, the options are optional. Most options have `long variants' also, which are mentioned in the following list. In this list, `-x, --optionname` are two alternate ways to specify option x. If `-x` takes an argument, it may be specified immediately following the `-x`, but separating blanks may also be used. Options not taking arguments can be combined (e.g., `-a -b -c` may be combined to `-abc`). Arguments specified with long options should be separated from the long option using a `=` character.

The following options are currently available:

- `-D, --define=NAME[=VALUE]`: Defines name as a symbol. This option is acts like `DEFINESYMBOL(NAME)()`. If `=VALUE` is added, `NAME` is initialized to `VALUE` (identically to `DEFINESYMBOL(NAME)(VALUE)`).
- `-d, --definemacro=NAME=EXPANSION`: Defines `NAME` as macro expanding to `EXPANSION`
- `-h, --help`: usage information is written to the standard error stream, describing all of Yodl's options.
- `-i, --index[=file]`: `file' is the name of the index file. By default `<outputbase>.idx` is used. No default when output is written to stdout. The index file is processed by Yodl's post-processor, `yodlpost`.
- `-I, --include=DIR`: This defines the system-wide include directory where `Yodl` searches for its input files. E.g. a statement to include a given file, like:

      INCLUDEFILE(latex)

  will cause Yodl to search for the file `latex` in the current directory, and when that fails, in the system-wide include directory. The system-wide include directory is typically the place where the maintainer of a system stores macro-files for Yodl. This searching process applies to files that are included inside a document but also applies to filenames on the command line when invoking the `Yodl` program.

  The name of the included file (`latex` in the above example) is the bare name, the `Yodl` program will supply a default extension (`.yo`), if necessary.

  The `-I` option overrules Yodl's built-in name for the system-wide include directory. The built-in name is defined when compiling Yodl, and is, e.g., `/usr/share/yodl`. Furthermore, the definition may contain $HOME, which will be replaced by the user's home directory if the `home' or `HOME' environment variable is defined. It may also contain $STD_INCLUDE, which will be replaced by the compilation defined standard include path. The standard includepath may be overruled by either (in that order) the command line switch `-I` or the `tt(Yodl)_INCLUDE_PATH` environment variable. By default, the current directory is added to the standard include path. Hewver, when `-I` or `tt(Yodl)_INCLUDE_PATH` is used, the current directory must be mentioned explicitly. The individual directories need not be terminated by a /-character. In distributed `.deb` archives, the standard directory is defined as `/usr/share/yodl` (prefixed by the current working directory).

- `-k, --keep-ws`: Since `Yodl` version 2.00 blanks at the begin and end of lines are ignored, even without a trailing \, when the `white space level' is non-zero. Earlier versions kept these blanks. The legacy handling of white space at end of lines can by obtained using the `-k` flag. Note that white space are always kept when using verbatim copying, and when the white-space level is zero.
- `-l, --live-data=HOW`: This option controls the policy for executing `SYSTEM` or `PIPETHROUGH` commands; HOW being `none` (0) by default. The HOW argument can have the following values:
  - `none` or 0: (the default): No macros calling system programs are allowed.
  - `confirm` or 1: The macros can be executed, but only after user confirmation is obtained. The macros in question are shown while the Yodl document is processed, and the user must either accept or reject the call.
  - `report` or 2: The macros are executed, but what is called is shown during the Yodl run (if the WARNING message level is active).
  - `ok` or 3: The macros are executed, and not shown during the run. Be careful when using `--live-data ok`. It should be used only when a document is clearly `unharmful'.
- `-m, --messages=SET`: Set the so-called `message level' to a combination of the SET `acdeinw`. The letters of this set have the following meanings:
  - `a`: alert. When an alert-error occurs, Yodl terminates. Here Yodl requests something of the system (like a `get_cwd()`), but the system fails.
  - `c`: critical. When a critical error occurs, Yodl terminates. The message itself can be suppressed, but exiting can't. A critical condition is, e.g., the omission of an open parenthesis at a location where a parameter list should appear, or a non-existing file in an `INCLUDEFILE` specification (as this file should be parsed). A non-existing file with a `NOEXPANDINCLUDE` specification is a plain (non-critical) error.
  - `d`: debug. Probably too much info, like getting information about each character that was read by Yodl.
  - `e`: error. An error (like doubly defined symbols). Error messages will not stop the parsing of the input (up to a maximum number of errors), but no output is generated.
  - `i`: info. Not as detailed as `debug', but still very much info, like information about media switches.
  - `n`: notice. Information about, e.g., calls to the builtin function calls.
  - `w`: warning. Something you should know about, but probably not affecting Yodl's proper functioning
  Non-configurable is the handling of an emergency message. These messages can't be suppressed, but shouldn't happen, as they point to some internal error. It would be appreciated to receive information about these messages if they ever occur.
- `-n, --max-nested-files=NR`: This option causes Yodl to abort when the number of nested input files exceeds `NR`, which is 20 by default. Exceeding this number usually means a circular definition somewhere in the document. This is the case when, a file `a.yo` includes `b.yo`, while `b.yo` includes `a.yo` etc.. It does not prevent recursive macro- or subst-replacements. For that the `-r` (`--max-replacements`) option is available.
- `-o, --output=FILE`: This option causes Yodl to write its output to `FILE`. By default, the output goes to the standard output stream. E.g., you can use `Yodl` to read a file `input` and to write to `output` with the following two commands:

              yodl input > output
              yodl -ooutput input

  The difference being that in the latter case an index file is generated, but not in the former case. Notice that writing an index file can be forced when the `--index` option is specified.
- `-p, --preload=CMD`: This option `pre-loads' the string `cmd`. It acts as though `cmd` was the first command in the first input file that is processed by `Yodl`.

  More than one `--preload=CMD` options may be present on the command line. Each of the commands is then processed in turn, before reading any file.

- `-r, --max-replacements=NR`: This option causes Yodl to abort when the number of macro calls or subst-replacements exceeds `NR * 10,000`. By default, `NR` equals 1. Setting `--max-replacements=0` implies that no macro- or subst-replacement checks are performed.
- `-t, --trace`: This option enables tracing: while parsing, Yodl writes its output to the standard error stream. As is the case with the `-k` option, this option is defined for debugging purposes only.
- `-V, --version`. This option will show `Yodl`'s actual version.
- `-v, --verbose`: This option increases Yodl's `verbosity level' and may occur more than once. By default yodl will show alerting, critical, emergency and error messages. Each `--verbose` option will add a next message level. In order, warning, notice, info and debug messages will be added to this set. It is also possible to suppress messages. The `VERBOSITY` builtin can be used for that.
- `-W, --warranty`. This option will show a warranty disclaimer and a copyright notice.
- `-w, --warn`: The presence of this option caused Yodl to warn when, e.g., symbols are redefined.

The inputfile elements on the command line specify which files Yodl should process. All names are supplied with an extension (this extension is defined in the installation of Yodl and is usually `.yo`). The files are then searched for in the directories mentioned in the include-path. Files may also be specified using absolute pathnames.

Note that all filenames on the command line are input files. To define an output file, either use the `--output` option or redirect the output.

## 2.2: The Yodl grammar

The grammar which is used by `Yodl` mixes `real' text that should appear on the output with markups: commands for `Yodl`. The markups must follow a certain grammar, which is described in this section. `Yodl` therefore falls in the category of `markup languages', in contrast to `WYSIWYG'-programs. As a consequence, `Yodl` promotes concept-oriented writing.

Basically, Yodl only does `something special' when it encounters the name of a builtin function or the name of a user-defined macro, followed by a parameter list. Sometimes a function or macro requires multiple arguments, which must then be specified in sequence. All required parameter lists, however, must be specified within the same input file. It is not allowed to split the activation of a builtin function or macro over multiple input files. Plain text, on the other hand, may be split over multiple files.

In this section the elements of `Yodl`'s grammar are briefly discussed.

### 2.2.1: Language elements

At the lowest level, `Yodl`'s lexical scanner returns small pieces of information to its parser. These pieces of information are called tokens, and consist of elements like a blank space, a non-blank character, or an end-of-ile flag. These tokens are at too small an aggregation level to be useful for the current user-guide, so here we concentrate our discussion on the next aggregation level: compound elements and conceptual elements.

Compound elements relate to the basic tokens as words in a sentence to the individual letters of the words. These compound elements are identifiers, names, numbers and parameter lists.

Conceptual elements are found at the next higher aggregation levels: builtin functions are the buildin blocks for all of `Yodl`'s functionality, symbols and counters are `Yodl`'s variables, and (user defined) macros extend `Yodl`'s functionality beyond those of the basic builtin functions.

In the coming sections these basic and conceptual elements are discussed in greater detail.

#### 2.2.1.1: Identifiers and Names

Identifiers are names that can have a special meaning in the `Yodl` language. E.g., the sequence `INCLUDEFILE` is an identifier: when followed by a filename in parentheses, `Yodl` will take some special action (in this case, read the file as a `Yodl`-source file).

Identifiers may consist of uppercase or lowercase characters. No other characters may appear in them.

In particular, note that this diverts from the well known definition for identifiers used in most programming languages: identifiers may not contain underscores, nor digits. `Yodl`, therefore, won't accept identifiers like `run4` or `under_score`.

Names are sequences of characters, not containing white space characters. (i.e., any series of characters not containing spaces, tabs or newlines). Names are allowed with certain builtin functions, liek the `INCLUDEFILE` function, expecting the name of a file as its argument.

#### 2.2.1.2: Numbers

Numbers consist of digits and an optional minus sign. They are most often used for so-called counters. In some contexts (e.g. with the builtin function VERBOSITY 3.1.73, hexadecimal numbers are allowed. Hexadecimal numbers have 16 `digits': the familiar 0-9, but also `a-f` (or `A-F`), representing the decimal values 10 until 15, respectively. Hexadecimal values are usually prefixed by `0x`, for example `0x4e`.

In other contexts (in particular, with character tables 2.3), octal numbers or character constants are allowed too.

An octal number only consists of the digits 0-7. In `Yodl`, octal values must consist of three digits, and must be preceded by a backslash.

Character constants may very well be considered numerical values. Character constants consist of a character value between single quotes, for example `'a'`.

Refer to section 2.3 for more detailed information about the use of octal values and character constants.

`Yodl` has no concept of floating point values nor does it have facilities for performing floating point arithmetic.

#### 2.2.1.3: Parameter lists

Parameter lists contain arguments to `Yodl` builtin functions or user-defined macros. Each parameter list contains exactly one argument, and must be enclosed by parentheses.

A parameter list is recognized as such when encountered immediately following the name of a builtin function or user-defined macro. Some functions or macros expect multiple arguments. In those cases, the required number of arguments must be provided, possibly separated from each other by white-space only.

For example, the following shows how to call the builtin function `DEFINECOUNTER`, expecting two arguments:

        DEFINECOUNTER(MyCounter)()
        DEFINECOUNTER(MyCounter)    ()
        DEFINECOUNTER(MyCounter)(12)

`Yodl` recognizes the arguments to a macro as parameter lists, i.e., delimited by `(` and `)`. As long as the numbers of opening and closing parentheses match, `Yodl` will correctly recognize the list. E.g., given a hypothetical macro `somemacro`, the following code sample shows the macro followed by one parameter list:

        somemacro(Here is a chunk of text.)
        somemacro(Here is a some (more) text.)

A problem arises when the number of parentheses is unbalanced: i.e., when the parameter list consists of more opening than closing parentheses or vice versa To handle such situations, Yodl offers a `literal-character' mechanism (see the `CHAR` macro in 3.1.4) and a `global substitution' mechanism (see the `SUBST` macro in 3.1.65). For example, to send the text

    here's a ")" closing parenthesis

as an argument to our hypothetical macro `somemacro`, the following can be used:

        COMMENT(-- Alternative 1: using CHAR --)
        somemacro(here's a "CHAR(41)" closing parenthesis)

        COMMENT(-- Alternative 2: using SUBST --)
        SUBST(closepar)(CHAR(41))
        somemacro(here's a "closepar" closing parenthesis)

Both methods have disadvantages: the `CHAR` method requires you to remember that an ASCII 41 is a closing parenthesis. The `SUBST` method defines a string `closepar` that is always expanded to a closing parenthesis, wherever it may occur in the text. But whatever method is used, it should be clear by now that unbalanced parameter lists can be handled by Yodl. Also, remember that unbalanced parenthesis pairs are only relevant in argument lists. Yodl handles parentheses in normal text as ordinary characters.

#### 2.2.1.4: Builtin functions

The building blocks of `Yodl`'s functionality are its builtin functions. Builtin functions exists to manipulate all of `Yodl`'s builtin types (character tables, counters, macros and symbols) and to do basic bookkeeping and flow-control: it is possible to test values of counters and symbols, to include other input files, to generate warning and error messages, and to start child- or subprocesses. Each builtin function is described in a separate subsection of section BUILTIN 3.1.

#### 2.2.1.5: Character translation tables

Character translation tables exist to perform conversion specific transformations. For example, in `html`, the \`'e` is written as `&eacute;`, but in LaTeX it's written as \`'e`. Rather than using a potentially long if-else ladder to determine how to set a particular character, a character translation table can be used. The character translation table of a particular conversion is then activated only for that type of conversion.

Character table translations are used very late during the processing of `Yodl`'s input s: it is the output generator that handles the character translations. Consequently, macros or builtin function calls that might appear in a character's redefinition in a character table will not be expanded. In practice this never is a point of concern. In section 2.3 the use of character translation tables is discussed in detail.

#### 2.2.1.6: Counters

Some document languages (notably LaTeX) automatically prefix numbers when typesetting sections, subsections, tables, figures etc.. Other document languages (e.g. `html`) don't.

Therefore, a macro package that converts a Yodl document to LaTeX doesn't need to provide the numbering of sections etc.. However, if you do want the numbering and if you want to convert documents to, say, `html`, then you must take care of the numbering yourself.

Counters exist to make this possible. Counters can be incremented, can be given a particular value, can be given a new value temporarily and can be removed. They always contain integral values, which may be negative.

Section 2.5 describes the use of counters in more detail.

#### 2.2.1.7: Macros

Macros are comparable to builtin functions, but they can be defined in `Yodl` input files. Macros add functionality to `Yodl` exceeding the basic functionality of the builtin functions. Macros can have arguments, and they are used in exactly the same way as builtin functions are used.

When `Yodl` encounters a macro, it acts as follows:

- Its arguments are obtained, by reading its argument lists. These arguments are not interpreted in any way. They are simply removed from the input, and stored for further processing;
- References to arguments in the macro's definition (using the `ARG#` notation, where `#` is the sequence number of a particular argument) are replaced by the literal text of the corresponding macro's arguments.
- The thus modified definition text is now pushed back into the input stream, to be processed by `Yodl`'s lexical scanner.

Defining macros is described in section 3.1.11. Macros may be defined, deleted, renamed, and temporarily given other definitions.

#### 2.2.1.8: Nousermacros

When `Yodl` is started using the `-w` flag on the command line, then warnings are generated when Yodl encounters a possible macro name, followed by a parameter list, without finding a macro by that name. Yodl then prints something like `cannot expand possible user macro`.

Examples of such sequences are, `The necessary file(s) are here`, or `see the manual page for sed(1)`. The candidate macros are `file` and `sed`, as these names could very well have been `valid' user macros followed by their parameter list.

A nousermacro can be defined to suppress these warnings, by informing `Yodl` that `file` and `sed` aren't macros. Nousermacros may be defined and undefined. See sections 3.1.45 and 3.1.16 for details).

#### 2.2.1.9: Symbols

`Yodl` symbols contain text. They were introduced to allow the flexible expansion of text, the length and/or content of which cannot be determined in advance. In particular, symbols are useful to store a series of LaTeX document options, or a series of `html` body options. In earlier versions of `Yodl` complex and confusing constructions using nested definitions of macros were used for this. These macros were not only confusingly complex, but they also suffered from a hard-coded maximum. Symbols solve these drawbacks, and now that they are available, they are used for all natural situations in which an initially unknown piece of text must be stored. National language specific strings are another useful area in which symbols can be used. The symbol `CONTENTSHEADING` can be set to the name of the contents heading (e.g., `Contents` in English, `Inhoud` in Dutch, `Contenido` in Spanish, and macros can simply insert the value of the symbol `CONTENTSHEADING` at the appropriate location.

Symbols can be defined 3.1.12, removed 3.1.17, (temporarily 3.1.59 or permanently 3.1.63) be given another value; pushed symbol values can be restored 3.1.54 at a later point. Of course, their values can also be inserted 3.1.66 into `Yodl`'s output file.

### 2.2.2: Line continuation

To make the typing of input easier, `Yodl` allows you to end a line with a backslash character \ and to continue it on the next line. That way you can split long lines to fit your screen. When processing its input, `Yodl` will treat these lines as one long line, and will of course ignore the \ character. This feature only works when the \ character is the last one on the line (no spaces may follow).

When the line following the one with the \ character has leading spaces, then these are omitted. This allows you to `indent' a file as you wish, while the space characters of the indentation are ignored by the `Yodl` program.

A trivial example is the following:

        Grandpa and\
        grandma are sitting on the sofa.

Due to the occurrence of the \ character in the sequence `and\`, `Yodl` will combine the lines to

        Grandpa andgrandma are sitting on the sofa.

Note that the spaces before `grandma` are ignored, since this is the second line following a \ character.

If you do want one or more spaces while joining lines with \, put the spaces before the \ character.
Summarizing:

- A Line ending in a backslash character is merged with the next line.
- This only happens if the \ character is the last character of the line, no spaces may appear behind the \.
- When merging lines, `Yodl` ignores leading spaces of the second line.

The question is of course, how do you accomplish that a line really ends with a \, when you do not want `Yodl` to merge it with the following line? In such a case, type a space character following your \: `Yodl` won't combine the lines. Or set the \ character as `CHAR(\)` or `CHAR(92)` (see section 3.1.4 for the `CHAR` macro).

When `Yodl` processes input files, and the white-space level exceeds zero (see section 3.1.38), then all lines are processed as if they terminated by a \. This behavior was implemented first with `Yodl` version 2.00. It can be suppressed using `Yodl`'s `-k` flag.

### 2.2.3: The +identifier sequence

There may be situations in which you must type a macro name right after a sequence of characters, while Yodl should recognize this. Imagine that someone wrote a great macro `footnote` for you (someone did, in fact, see the next chapter), to typeset footnotes. If you'd type in a document:

    The C Programming Languagefootnote(as defined by
    Kernighan and Ritchie) ...

then of course Yodl would fail to see the start of a macro in the sequence `Languagefootnote`. You could say

    The C Programming Language footnote(as defined by
    Kernighan and Ritchie) ...

but that would introduce a space between `Language` and the footnote. Probably you don't want that, since spaces between a word and a footnote number look awful and because of the fact that the footnote number might be typeset on the following line.

For these special situations, Yodl recognizes the `+identifier` sequence as the start of a macro, while the `+` sign is effectively ignored. In the above example you could therefore use

    The C Programming Language+footnote(as defined by
    Kernighan and Ritchie) ...

The `+identifier` recognition only works when the identifier following the `+` sign is a macro. In all other situations, a `+` is just a plus-sign.

(The `+identifier` sequence furthermore plays an important role in macro packages. If you're interested, see the file `shared.yo` which is by default installed to `/usr/local/lib/yodl`.)

### 2.2.4: Preventing macros from being expanded

One more feature of the Yodl language remains to be described. In the previous section it was described how a macro may be called immediately following alphabetical characters. What about the opposite situation where we do not want a macro to be expanded in a particular situation? The `NOUSERMACRO` builtin command (cf. section 3.1.45) may be used to suppress the interpretation of a character sequence (e.g., `file(...)`) as a macro, but what if a macro should not be expanded in the occasional situation? For this case various solutions are available:

- First, the `tt(...)` and `verb(...)` macros may be used to suppress macro expansion. These macros will also temporarily change the typesetting font, though.
- Second, `NOEXPAND()` builtin command may be used: the macro name may be passed to `NOEXPAND()`, immediately followed by the `argument list':

      Like this: NOEXPAND(NOEXPAND)(hello world)

- Third, the `nop()` macro may be used to separate a macro name from its argument list:

      Like this: NOEXPAND+nop()(hello world)

## 2.3: Character tables

The Yodl language provides a way to define character translation tables, to activate them, and to deactivate them. A character translation table defines how a character in the input will appear in the output.

There are two main reasons for the need of character translation tables. First, a document language becomes much easier to use when you can type an asterisk as * instead of `$*$` or `\verb/*/` (these are sequences from the LaTeX document language). Hence, a mechanism that expands a * in the input to to `\verb/*/` on the output, saves the users a lot of typing.

Second, forcing users to type weird sequences won't work if you're planning on converting the same Yodl document to a different output format. If the user types `\verb/*/` in the input to typeset an asterisk in the output, how should he or she arrive at a single * in the output in another output format?

The solution is of course to define the translation for an input character like * given the output format.

### 2.3.1: Defining a translation table

The built-in macro `DEFINECHARTABLE` defines a character translation table. It takes two parameter lists: the name of the table and the character translations. Hence, each table is defined by its own name.

As an example of a table, consider the following fragment. It defines a table that translates the upper case characters `A` to `E` to their lower case equivalents:

    DEFINECHARTABLE(tolower)(
        'A' = "a"
        'B' = "b"
        'C' = "c"
        'D' = "d"
        'E' = "e"
    )

Each `DEFINECHARTABLE` statement must have a non-empty second parameter. "Empty" character tables cannot be defined, though one non-translation table is built-in.

The syntaxis of the second parameter list is as follows:

- On separate lines, input characters are mapped to a sequence to appear on the output.

- Per line, the input character is specified as `'c'`, `c` being any character. Escape-sequences from the C programming language can be used in this specification; Yodl supports the sequences `\a` (alert), `\b` (beep), `\f` (formfeed), `\n` (newline), `\r` (carriage return), `\t` (tab), and `\v` (vertical tab). Octal and hexadecimal constants may also be used. E.g., character `Y` may also be specified using the octal value `\131` or the hexadecimal value `\x59`. Any other character following a \ defines itself: `\\` represents a single backslash character.

- Following the character specification, a `=` must appear.

- Following that, a sequence of one or more characters appears, enclosed in double quotes, defining the translation. Again, escape sequences can be used, as in:

      '\n' = "End of line\n"

  Such a mapping adds the text `End of line` to each line, since each newline character in the input is replaced by the text `End of line`, followed by the newline itself.

  Starting with Yodl 2.14.0 octal and hexadecimal constants may also be used within the double quoted string. E.g., character `Y` may also be specified using the octal value `\131` or the hexadecimal value `\x59`. As an example where the octal/hexadecimal values may be useful consider the processing of a man-page. The character representations for the literal double quote (`"`) in `troff` is `\(dq\&`. However, since `(` cannot be written literally in the character translation table since that would result in unbalanced parentheses while processing the character table's definition. Also, `CHAR``(40)` cannot be used, since character table conversiond are performed by the output generator, which is called after the macro expansions have been performed. This it would result in the literal text `CHAR`((40)) appearing in the manual page.

  Using the octal character representation in the chartable specification for the `"` character appearing in man-page the problem can now be solved. The actual specification used is:

       '"' = "\\\050dq\\&"

Translations which are not specified in the table are left to the default, which is to output the character as-is.

Note that the character table translation is something that the `yodl` program does as one of its last actions, just before sending text to the output file. The expansion text is not further processed by `yodl`, except for the conversion of C-type escape sequences to ordinary characters. The expansion text should therefore not be protected by, e.g., `NOTRANS` (unless of course you want some character to generate the text `NOTRANS` on the output).

### 2.3.2: Using a translation table

A defined translation table is activated by the macro `USECHARTABLE`. This macro takes one parameter list, which may be:

- empty, in which case the default mapping is restored,

- a name of a previously defined character table.

The default mapping, selected when an empty parameter list is given, means that Yodl enters its `zero translation state', meaning no character translation at all.

### 2.3.3: Pushing and popping character tables

Besides the previously described macro `USECHARTABLE()`, Yodl has one other mechanism of activating and deactivating character translation tables. This mechanism uses a stack, and hence, the related macros are appropriately named `PUSHCHARTABLE()` and `POPCHARTABLE()`.

- `PUSHCHARTABLE(name)` pushes the currently active translation table onto a stack, and activates the table identified by `name`. The argument may be emtpy; in that case, the zero-translation table is activated (analogously to `USECHARTABLE()`).

- `POPCHARTABLE()` activates the translation table that was last pushed. There is no argument to this macro.

Using the push/pop mechanism is handy when a table must be temporarily activated, but when it is not known which table exacty is active prior to the temporary activation. E.g., imagine that you need to use a character table called `listing` to typeset a listing, but that you do not know the current table. The pushing and popping mechanism is then used as follows:

    COMMENT(First, we save the current table on the stack and
            we activate our "listing" table.)
    PUSHCHARTABLE(listing)

    COMMENT(Now the text is question is typeset.)
    ...

    COMMENT(The previously active table is re-activated, whatever its name.)
    POPCHARTABLE()

## 2.4: Sending literal text to the output

The Yodl program has three built-in macros to send literal text to the output file. The macros are listed in the above section 3.1 and are furthermore described here.

- The `CHAR` macro takes one argument: the ASCII number of a character or the character itself. The character is sent to the output file without being translated with the currently active character translation table.
- The `NOTRANS` macro takes one argument: the text in question. The text is neither parsed (i.e., macros in it are not expanded), nor translated with the current character translation table.
  The `NOTRANS` macro is conceptually like a series of `CHAR` macros.
- The `NOEXPAND` macro takes one argument: the text in question. The text is not parsed, but it is translated with the current character translation table.

To illustrate the need for the distinction between `NOTRANS` and `NOEXPAND`, consider the following. The `HTML` converter (described in chapter 4) must be able to send HTML commands to the output file, but must also be able to send literal text (e.g., a source file listing). The HTML commands of course must be neither translated with any character table, nor must they be expanded in regard to macros. In contrast, a source file listing must be subject to character translations: the `&`, `<` and `>` characters can cause difficulties. Two possible macros for a HTML converter are:

        COMMENT(--- htmlcommand(cmd) sends its argument as a HTML command
                    to the output ---)
        DEFINEMACRO(htmlcommand)(1)(NOTRANS(ARG1))

        COMMENT(--- verb(listing) sends the listing to the output ---)
        DEFINECHARTABLE(list)(
            '&'     =   "&amp;"
            '<'     =   "&lt;"
            '>'     =   "&gt;"
        )

        DEFINEMACRO(verb)(1)(
            USECHARTABLE(list)
            NOTRANS(<listing>)
            NOEXPAND(ARG1)
            NOTRANS(</listing>)
            USECHARTABLE(standard)
        )

In this example it is assumed that a character translation table `standard` exists, defining the `normal' translations. This table is re-activated in the `verb` macro.

## 2.5: Counters

Some document languages (notably LaTeX) automatically prefix numbers when typesetting sections, subsections, tables, figures etc.. Other document languages (e.g. HTML) unfortunately don't.

Therefore, a macro package that converts a Yodl document to LaTeX doesn't need to provide the numbering of sections etc.. However, if you do want the numbering and if you want to convert documents to, say, HTML, then you must take care of the numbering yourself.

This section describes the counters in Yodl: how to create counters, how to use them, etc..

### 2.5.1: Creating a counter

Before a counter can be used, it must be created with the function `DEFINECOUNTER` or `PUSHCOUNTER`. These functions expects two parameter lists: the name of the counter and an optional value.

The counter's value, named `number` below, may be set as follows:

- If left unspecified, the counter is set to 0;
- `number` may be a postive or negative integral value;
- `number` may be the name of an existing counter, in which case that counter's value is used.

For example, let's say that our macro package should provide two sectioning commands: `section` and `subsection`. The sections should be numbered 0, 1, 2, etc., and the subsections 1.1, 1.2, 1.3 etc.. Hence we'd need two counters:

        DEFINECOUNTER(sectcounter)()
        DEFINECOUNTER(subsectcounter)(1)

The function `NEWCOUNTER`, as defined in earlier releases of `Yodl`, is still available, but is deprecated.

### 2.5.2: Using counters

The builtin function `COUNTERVALUE(somecounter)` expands to the value of `somecounter`. E.g., if the current value is 2, then the value 2 is inserted into the output object. It is an error to use `COUNTERVALUE` on a non-existing counter or on a counter not having a defined value (see below).

Yodl has several functions to modify and/or to set the values of counters. The counter's value, named `number` below, may be set as follows:

- If left unspecified, the counter is set to 0;
- `number` may be a postive or negative integral value;
- `number` may be the name of an existing counter, in which case that counter's value is used.

The functions modifying values of counters are:

- `POPCOUNTER(somecounter)`: This function pops the most recently pushed value off the counter's stack, assigning it to `somecounter`. An error occurs when `somecounter` doesn't exist. If the counter was never pushed, it will still exist following `POPCOUNTER`, but its value is undefined: using `COUNTERVALUE(somecounter)` in that case generates an error.
- `PUSHCOUNTER(somecounter)(number)`: This function pushes the current value of the counter `somecounter` on the counter's stack, making `number` its new value. `number` may be left unspecified, in which case the counter will be set to 0. When `somecounter` doesn't exist yet, it is created with an initial value of `number`.
- `SETCOUNTER(somecounter)(number)`: This function sets the value of `somecounter` to the value of `number`. The second parameter list must be an integer number (i.e., consisting of the characters `0` to `9`, optionally prefixed by a `-` sign). The function does not expand to anything; i.e., it does not write to the output file.
- `ADDTOCOUNTER(somecounter)(number)`: This function adds the value of `number` to `somecounter`. The number may be negative.
- `USECOUNTER(somecounter)`: This function first increases the value of `somecounter` by 1, and then writes the value of the counter to the output file.

  This function is particularly useful in combination with `DEFINECOUNTER`: since `DEFINECOUNTER` initializes a counter to zero, `USECOUNTER` can be used to increase the value and to output it. The first time that `USECOUNTER` is used on a new counter, the number 1 appears on the output file. The next time, number 2 appears on the output file etc..

Given the numbering requirements of the hypothetical commands `section` and `subsection` (see the previous section), we can now complete the definitions:

    DEFINECOUNTER(sectcounter)
    DEFINECOUNTER(subsectcounter)

    DEFINEMACRO(section)(1)(\
    SETCOUNTER(subsectcounter)(0)\
    USECOUNTER(sectcounter) ARG1)

    DEFINEMACRO(subsection)(1)(\
    COUNTERVALUE(sectcounter).USECOUNTER(subsectcounter) ARG1)

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