parsing - ANTLR rewrite rules in grammar file -

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i have rule looks this:

a : (b | c) d;  b : 'b';  c : 'c';  d : 'd';

with grammar antlr builds flat parse tree. how can rewrite first rule (and leave other 2 unchanged) whatever matched being returned under root node called a?

if first production rule this:

a : b d;

then have been rewritten 

a : b d -> ^(a b d)

and have solved problem. first grammar rule yields more 1 possibility resulting parse tree ^(a b d) or ^(a c d).

how express when rewriting rule?

you can use ? operator in rewrite follows.

a : (b | c) d -> ^(a b? c? d);

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