Hash.purge

immutable data = ["foo": 1];
immutable subject = new immutable(Hash!(string, int))(data);
auto result = subject.insert("bar", 2).purge;
assert(result._data["foo"] == 1);
assert(result["foo"] == 1);
assert(result._data["bar"] == 2);
assert(result["bar"] == 2);

assert(result._previousVersion is null);