anchor.diff:  -spec fold_extract_op(key(), riakc_datatype:datatype(), [field_update()]) -> [field_update()].
   fold_extract_op(Key, Value, Acc0) ->
       Mod = type_module(Key),
  -    case Mod:to_op(Value) of
  -        undefined ->
  -            [{add, Key}|Acc0];
  -        {_Type, Op, _Context} ->
  -            [{update, Key, Op} | Acc0]
  -    end.
  +    {_Type, Op, _Context} =  Mod:to_op(Value),
  +    [{update, Key, Op} | Acc0].
   
   -ifdef(EQC).
   gen_type() ->
  
positive.code: 
  gen_op() ->
      oneof([
             {add, [gen_key()]},
             {erase, [gen_key()]},
             ?LAZY({update, ?LET({_,T}=Key, gen_key(), [Key,
                                                        gen_update_fun(T)])})
  
negative.code:          type/0]).
  
  %% Operations
  -export([erase/2,
           update/3]).
  
  %% Queries
  