Goal:
  
  A : Type
  H : ReflectEq A
  x : A
  xs : list A
  H0 : is_prefix [] (x :: xs)
  ============================
   ([] = [])


DEBUG:Going to execute:
let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"