anchor.diff:              ),
               gen_w1c()
           },
  -        Defaults ++ Mult ++ DatatypeOrConsistent ++ FastPath).
  +        Defaults ++ Mult ++ DatatypeOrConsistent ++ WriteOnce).
   
   gen_allow_mult() ->
       ?LET(Mult, frequency([{9, bool()}, {1, binary()}]), [{allow_mult, Mult}]).
  
positive.code: validate_update_type(consistent, Existing, New) ->
      validate_update_consistent_props(Existing, New);
  validate_update_type(write_once, _Existing, New) ->
      NewFastPath = proplists:get_value(write_once, New),
      validate_update_w1c_props(NewFastPath, New);
  validate_update_type(datatype, Existing, New) ->
      validate_update_dt_props(Existing, New);
  validate_update_type(default, _Existing, New) ->
  
negative.code: %    oneof([gen_valid_w1c(), {write_once, rubbish}]).
  
  gen_w1c() ->
      ?LET(WriteOnce, frequency([{9, bool()}, {1, binary()}]), [{write_once, WriteOnce}]).
  
  gen_valid_w1c() ->
      ?LET(WriteOnce, bool(), [{write_once, WriteOnce}]).
  
  %% helpers
  
  