PrimitiveParser is the most basic building block that the client uses to build more complex parsers.
Upon entry to the
member function, a PrimitiveParser is required to do a pre-skip. Leading
will be skipped prior to parsing. Only PrimitiveParsers are required
to perform this pre-skip. This is typically carried out through a call
qi::skip_over(f, l, skip);
Metafunction that evaluates to