temporary commit -- problem with or operator in parser combinators is that for a rule r <- a | b, if a matches, b will never be tried. with precedence, a & b should be parser by the and rule and a & b -> c should be parser by the implication rule, but not both of them will be tried by the actual implementation. atm looking for a way to combine common prefixes.

git-svn-id: http://gapt.googlecode.com/svn/trunk/source@1375 85d21ef8-add0-11de-ae77-5bfb9c5a3f34