anchor.diff:      ?FORALL(
          {Schema, FVs}, schema_data_fields(),
          begin
  +           %% Schema is a generated #shema_node{} record
              Root = root(Schema),
              Obj = ecapnp_obj:from_ref(Root, Schema),
              lists:foldl(
  
positive.code:       ]).
  
  %% struct ref with at least one word of data
  struct_ref_data() -> struct_ref(range(1, inf), range(0, inf)).
  %% struct ref with at least one pointer
  struct_ref_ptr() -> struct_ref(range(0, inf), range(1, inf)).
  
  %% any struct ref, even null
  struct_ref() -> struct_ref(range(0, inf), range(0, inf)).
  
negative.code: %% property types
  %%% ----------------------------------------
  
  %% generate any kind of primitive value that capnp can represent.
  data_value() ->
      union(
        [?LET(B, boolean(), {1, bool, B}),
  