anchor.diff:  		{ok, Value} ->
   			filter_constraints(Tail, Map, Key, Value, Constraints);
   		error ->
  -			filter(Tail, maps:put(Key, Default, Map))
  +			filter(Tail, Map#{Key => Default})
   	end;
   filter([Key|Tail], Map) ->
   	true = maps:is_key(Key, Map),
  
positive.code: 		true ->
  			filter(Tail, Map);
  		{true, Value2} ->
  			filter(Tail, maps:put(Key, Value2, Map))
  	end.
  
  %% Tests.
  
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)
  