I am not so sure that manually building a tree that would work with
listeners/visitors generated from a second grammar is going to be an
option. I have asked on SO and on the Antlr discussion group and basically
got no responses as to how that might be possible. See
https://groups.google.com/forum/#!topic/antlr-discussion/vBkwCovqHcI
So the question is whether generating a semantic tree that is not Antlr
specific is a viable alternative. I think it is. And we can still provide
hand written listener and or visitor for processing this.