alt: alt combinator is similar to alternation in BNF. the parser
(alt(p1, p2)) recognises anything that p1 or p2 would.
The approach taken in this parser follows (Fairbairn86), in which either is
interpretted in a sequential (or exclusive) manner, returning the result of
the first parser to succeed, and failure if neither does.
Description
%alt% is the infix notation for the alt function, and it is the
preferred way to use the alt operator.
Usage
alt(p1, p2)
Arguments
p1
the first parser
p2
the second parser
Value
Returns the first parser if it suceeds otherwise the second parser