anchor.diff:  
   precondition(_, _) -> true.
   
  -postcondition(_S, {call,_,pick_backend,[Vips]}, Result) ->
  +postcondition(_S, {call, _, pick_backend, [Vips]}, Result) ->
     pick_backend_postcondition(Vips, Result);
   postcondition(_, _, _) -> true.
   
  
positive.code:                           observe_internal(0, Clock(), AccIn)
                        end,
                        Ewma,
                        [1,2,3,4,5,6,7,8,9,10]),
    cost(#backend{ewma = Later}) =< Initial.
  
  initial_state() ->
  
negative.code:                                                 clock = fun () -> Now end}
                                     end, [Higher, Lower]),
    %% We use inclusive bounds on both ends because large numbers
    %% (penalty is 1.0e307) will compare like this:
    %%    > 1.0e307 + 1 =:= 1.0e307.
    %%    true
    cost(BackendA) >= cost(BackendB).
  