If p1 and p2 are two parsers, p1 ||| p2 is a new parser that first tries
to parse the input using p1, and if that fails with exception Noparse,
tries p2 instead. The output is whatever parse result was achieved together
with the unparsed input.
FAILURE CONDITIONS
Never fails.
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.