Unresolved conflict for symbol sequence:

  '!'  expression  •  '<'  …

Possible interpretations:

  1:  (unary_a  '!'  expression)  •  '<'  …  (precedence: 2)
  2:  (unary_b  '!'  expression)  •  '<'  …  (precedence: 2)

Possible resolutions:

  1:  Specify a higher precedence in `unary_a` than in the other rules.
  2:  Specify a higher precedence in `unary_b` than in the other rules.
  3:  Add a conflict for these rules: `unary_a`, `unary_b`
