anchor.diff:               fun (_, false) -> false;
                    ({F, V}, true) ->
                        ok = ecapnp:set(F, V, Obj),
  -                     V =:= ecapnp:get(F, Obj)
  +                     compare_value(V, ecapnp:get(F, Obj))
                end, true, FVs)
          end).
   
  
positive.code: 
  %% create new message and get a ref to the root element
  root(A1, A2) -> root(data(A1, A2)).
  
negative.code:        {Def, {Off, Size, Value}}, struct_data(),
         begin
             Root = root(Def),
             %% note: value is binary
             ok = ecapnp_ref:write_struct_data(Off, Size, Value, Root),
             Value =:= ecapnp_ref:read_struct_data(Off, Size, Root)
         end).
  