\DOC |||

\TYPE {(|||) : ('a -> 'b) -> ('a -> 'b) -> 'a -> 'b}

\SYNOPSIS
Produce alternative composition of two parsers.

\DESCRIBE
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
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.

\SEEALSO
++, >>, a, atleast, elistof, finished, fix, leftbin, listof, many, nothing,
possibly, rightbin, some.

\ENDDOC