[antlr-interest] Getting rid of class cast exceptions to CommonTree

[Felix Dorner]

Hi,

I use a lot of semantic predicates in my tree grammar. When a 
FailedPredicateException is thrown, I sometimes end up with a ClassCast 
exception, because Antlr tries to cast an "UP" or a "DOWN" node, to my 
custom AST type. It seems that UP/DOWN are always CommonTrees.

I don't know where things get messed up, for now I'd just like to get 
rid of the class cast exception.

Felix