Parser::MGC class provides a low-level framework for building recursive-descent
parsers that consume a given input string from left to right, returning a parse
structure. It takes its name from the m//gc regexps used to implement the token
parsing behaviour.

It provides a number of token-parsing methods, which each extract a grammatical
token from the string. It also provides wrapping methods that can be used to
build up a possibly-recursive grammar structure, by applying a structure around
other parts of parsing code.