anchor.diff:  %% ===================================================================
   %% internal functions
   %% ===================================================================
  +
  +lookup_field(Name, Fields) ->
  +    proplists:get_value(Name, Fields, {unknown_field, Name}).
  
positive.code:                            }}=Object)
    when is_atom(Field) ->
      ecapnp_get:field(
        proplists:get_value(Field, Fields),
        Object
       ).
  
  
negative.code:                                   }}=Object)
    when is_atom(Field) ->
      ecapnp_set:field( 
        lookup_field(Field, Fields),
        Value, Object
       ).
  
  