Question

How can I solve mutual recursivity in this case?

value : comparison | 'A' | 'B' | 'C';

comparison : value comparison_operator value;

Answer 1

Well, the problem is that its ambiguous as given. What do you want it to do for an input like A <op> B <op> C where <op> is something that matches your comparison_operator rule? If you want the equivalent of A <op> (B <op> C) then its quite easy:

primary: 'A' | 'B' | 'C';
comparison: primary | primary comparison_operator comparison;
value: comparison;

If you want something else, it becomes more complex.