anchor.diff:  
   gen_op() ->
       oneof([
  -           {add, [gen_key()]},
              {erase, [gen_key()]},
              ?LAZY({update, ?LET({_,T}=Key, gen_key(), [Key,
                                                         gen_update_fun(T)])})
  
positive.code: %% <li>Like sets, removals will be processed before additions and
  %% updates in Riak. Removals performed without a context may result in
  %% failure.</li>
  %% <li>Adding or updating an entry followed by removing that same
  %% entry will result in no operation being recorded. Likewise,
  %% removing an entry followed by adding or updating that entry will
  %% cancel the removal operation.</li>
  %% </ul>
  %% @end
  
negative.code: 
  
  %% ==== Operations ====
  
  %% @doc Removes a key and its value from the map. Removing a key that
  %% does not exist simply records a remove operation. Removing a key
  