anchor.diff:  eval_rel_op(Bif, Args, #st{rel_op=Prev}) ->
       case normalize_op(Bif, Args) of
           none ->
  -            eval_boolean(Prev, Bif, Args);
  +            none;
           RelOp ->
               case will_succeed(Prev, RelOp) of
                   yes -> #b_literal{val=true};
  
positive.code:     case eval_bif(I, St) of
          #b_literal{}=Val ->
              eval_is(Is, From, bind_var(Dst, Val, Bs), St);
          failed ->
              eval_is(Is, From, bind_var(Dst, failed, Bs), St);
          none ->
              eval_is(Is, From, Bs, St)
      end;
  
negative.code:                 maybe -> none
              end
      end.
  
  %% will_succeed(PrevCondition, Condition) -> yes | no | maybe
  %%  PrevCondition is a condition known to be true. This function
  