Helper class which implements complex Map operations
in term of basic ones (Map.keys, Map.operator ,
Map.operator = and Map.remove.) Not all methods are
necessary to implement each particular operation.
Returns a string representing the specified map. The returned string
looks like this: '{key0: value0, key1: value1, ... keyN: valueN}'.
The value returned by its toString method is used to represent each
key or value.