Javadoc
Return a mapping from nondeterministc alt to combined list of predicates.
If both (s|i|semCtx1) and (t|i|semCtx2) exist, then the proper predicate
for alt i is semCtx1||semCtx2 because you have arrived at this single
DFA state via two NFA paths, both of which have semantic predicates.
We ignore deterministic alts because syntax alone is sufficient
to predict those. Do not include their predicates.
Alts with no predicate are assumed to have {true}? pred.
When combining via || with "true", all predicates are removed from
consideration since the expression will always be true and hence
not tell us how to resolve anything. So, if any NFA configuration
in this DFA state does not have a semantic context, the alt cannot
be resolved with a predicate.
If nonnull, incidentEdgeLabel tells us what NFA transition label
we did a reach on to compute state d. d may have insufficient
preds, so we really want this for the error message.