[antlr-interest] Pruning malformed ASTs

[Richard Thrippleton]

I am in the slightly painful situation of wanting to apply a tree grammar 
(type and other constraints checking) after a parser grammar even in the 
case that there were parse errors.

At the moment I just have the tree grammar exit at the moment that it 
encounters a malformed part of the tree; as I've seen stated on here before, 
there's no other sensible thing to do.

Sometimes I'm fortunate to have simple parse errors that still result in a 
correct tree (e.g. standalone unexpected tokens just get removed in 
recovery). For the larger parse errors I'll get an AST containing 
CommonErrorNodes which obviously trip up the tree grammar.

What I'm looking for is some common pattern or part of the ANTLR API that 
will allow the parser to snip subtrees at a specific granularity that 
contain error nodes as I create them. For example:

procedure:
	command* -> ^(PROC command*)

needs to ignore any tree coming out of the 'command' rule that contains 
error nodes.

Thanks,
Richard
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