I wrote the following pattern
            ^(e=entity{value=$e} X?)
 
Antlr produced the following logic
 
            e=entity();
            if( input.LA(1) == Token.DOWN){
                        match(input Token.DOWN, null);
                        value = e;.
            ......
                        match(input Token.UP, null);
            }
 
In other words, Antlr was perfectly willing to deal with the possibility that no X means no tree. But the action was only done if there was a tree.