This base 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. Each method, both token and
structural, atomically either consumes a prefix of the string and returns
its result, or fails and consumes nothing. This makes it simple to
implement grammars that require backtracking.