Goal:
    A : Type  H : ReflectEq A  x : A  l1 : list A  l2 : list A  l : list A
  ============================
   (x :: l1 ++ l = (x :: l1) ++ l)


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