The call a x gives a parser that parses a single item that is exactly x,
raising Noparse if the first item is something different.
FAILURE CONDITIONS
The call a x never fails, though the resulting parser may raise Noparse.
COMMENTS
This is one of a suite of combinators for manipulating ``parsers''. A parser is
simply a function whose OCaml type is some instance of
:('a)list -> 'b * ('a)list. The function should take a list of objects of
type :'a (e.g. characters or tokens), parse as much of it as possible from
left to right, and return a pair consisting of the object derived from parsing
(e.g. a term or a special syntax tree) and the list of elements that were not
processed.