Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  ls2 : list A
  ls3 : list A
  H : [] ≠ []
  ============================
   (#|ls2| = #|ls3|)


DEBUG:Going to execute:
(solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"