Some words of explanation about the `set`/`unset`/`isSet` methods: SBML Levels 1 and 2 define certain attributes on some classes of objects as optional. This requires an application to be careful about the distinction between two cases: (1) a given attribute has never been set to a value, and therefore should be assumed to have the SBML-defined default value, and (2) a given attribute has been set to a value, but the value happens to be an empty string. LibSBML supports these distinctions by providing methods to set, unset, and query the status of attributes that are optional. The methods have names of the form `set` Attribute`(...)`, `unset` Attribute`()`, and `isSet` Attribute`()`, where Attribute is the the name of the optional attribute in question.