anchor.diff:          with
           | Failure _ -> 
               let th = conv tm
  -            net := Choice.get <| enter [] (tm, ALPHA_HACK th) (!net)
  +            match enter [] (tm, ALPHA_HACK th) (!net) with
  +            | Success n -> net := n
  +            | Error _ -> 
  +                // NOTE: currently do nothing in case of error
  +                ()
               th
  
positive.code: 
  /// Symmetry conversion.
  let SYM_CONV tm = 
      let th1 = SYM(ASSUME tm)
      let tm' = concl <| Choice.get th1
      let th2 = SYM(ASSUME tm')
      DEDUCT_ANTISYM_RULE th2 th1
      |> Choice.mapError (fun _ -> Exception "SYM_CONV")
  
  (* ------------------------------------------------------------------------- *)
  (* Conversion to a rule.                                                     *)
  (* ------------------------------------------------------------------------- *)
  
  /// Conversion to a rule.
  let CONV_RULE (conv : conv) th = EQ_MP (conv(concl <| Choice.get th)) th
  
  (* ------------------------------------------------------------------------- *)
  (* Substitution conversion.                                                  *)
  
negative.code: let SUBS_CONV ths tm = 
      let tm = 
          if ths = [] then REFL tm
          else
              choice { 
                  let! lefts = Choice.List.map (Choice.bind lhand << Choice.map concl) ths
                  let! gvs = Choice.List.map (Choice.map genvar << type_of) lefts
                  let! pat = subst (zip gvs lefts) tm
                  let abs = list_mk_abs(gvs, pat)
                  let th = 
                      rev_itlist (fun y x -> CONV_RULE (THENC (RAND_CONV BETA_CONV) (LAND_CONV BETA_CONV)) (MK_COMB(x, y))) 
                          ths (REFL abs)
                  let! tm' = Choice.bind (rand << concl) th
                  if tm' = tm then return! REFL tm
                  else return! th
              }
      tm 
      |> Choice.bindError (fun _ -> Choice.failwith "SUBS_CONV")
  
  (* ------------------------------------------------------------------------- *)
  (* Get a few rules.                                                          *)
  