anchor.diff:  		true ->
   			filter(Tail, Map);
   		{true, Value2} ->
  -			filter(Tail, maps:put(Key, Value2, Map))
  +			filter(Tail, Map#{Key => Value2})
   	end.
   
   %% Tests.
  
positive.code: 		{ok, Value} ->
  			filter_constraints(Tail, Map, Key, Value, Constraints);
  		error ->
  			filter(Tail, maps:put(Key, Default, Map))
  	end;
  filter([Key|Tail], Map) ->
  	true = maps:is_key(Key, Map),
  
negative.code: 					case maps:find(Atom, Map) of
  						{ok, MapValue} when is_list(MapValue) ->
  							kvlist_to_map(Keys, Tail,
  								Map#{Atom => [Value|MapValue]});
  						{ok, MapValue} ->
  							kvlist_to_map(Keys, Tail,
  								Map#{Atom => [Value, MapValue]});
  						error ->
  							kvlist_to_map(Keys, Tail,
  								Map#{Atom => Value})
  					end;
  				false ->
  					kvlist_to_map(Keys, Tail, Map)
  