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


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