SBML Level 1 uses a text-string format for mathematical formulas. SBML Level 2 uses MathML, an XML format for representing mathematical expressions. LibSBML provides an Abstract Syntax Tree API for working with mathematical expressions; this API is more powerful than working with formulas directly in text form, and ASTs can be translated into either MathML or the text-string syntax. The libSBML methods that accept text-string formulas directly (such as this constructor) are provided for SBML Level 1 compatibility, but developers are encouraged to use the AST mechanisms.