ANTLR4 self and mutual left-recursion

Question

Is there a simple transformation or workaround to make this work in ANTLR4?

a : a p
  | b q
  | c
  ;

b : b r
  | a s
  | d
  ;

That is, a and b are self-left-recursive and mutual-left-recursive, the other rules (c, d, p, q, r, s) are just placeholders for simple rules.

Answer 1

Firstly, remove direct left recursion from both rules. Let's consider the rule a:

a
    : (b q | c) p+
    | b q
    | c
    ;

Simplify:

a
    : (b q | c) p*
    ;

Do a similar transformation for the rule b:

b
    : (a s | d) r*
    ;

Now it's possible to replace b identifier in rule a with the body of rule b:

a
    : (c | (a s | d) r* q) p*
    ;

Or to replace a identifier in rule b with the body of rule a:

b
    : ((b q | c) p* s | d) r*
    ;

It's also possible to get rid of direct left recursion if needed.