anchor.diff:  
       | _ -> MisMatch(delta_depth_of_term env t1, delta_depth_of_term env t2)
   
  -(* Does t1 match t2, after some delta steps? *)
  +(* Does t1 head-match t2, after some delta steps? *)
   let head_matches_delta env wl t1 t2 : (match_result * option<(typ*typ)>) =
       let maybe_inline t =
           let head, _ = U.head_and_args t in
  
positive.code:     | MisMatch(i, j) -> MisMatch(i, j)
      | _ -> HeadMatch
  
  let and_match m1 m2 =
      match m1 with
      | MisMatch (i, j) -> MisMatch (i, j)
  
negative.code:             | _ -> success n_delta r t1 t2 in
      let r = aux true 0 t1 t2 in
      if Env.debug env <| Options.Other "RelDelta" then
          BU.print4 "head_matches_delta (%s, %s) = %s (%s)\n"
              (Print.term_to_string t1)
              (Print.term_to_string t2)
              (string_of_match_result (fst r))
  