Parse a string as described by the grammar. The format of the
grammar is described in the document called 'Parsing'. If parsing
is successful, the parse tree is returned as an array. The optional
third argument specifies the number of alternative parse trees to
integrate in the result, if the grammar is ambiguous.
parse_string() uses internal object
storage to cache grammars and generated automatons, which is not
removed until the object is destructed.
This function cannot be used from a user or editor object.
