Hash.opApply

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

V[] values;
K[] keys;

foreach(K key, V value; subject) {
    values[values.length++] = value;
    keys[keys.length++] = key;
}

assert(values.length == 2);
assert(keys.length == 2);

immutable data = ["foo": 1];
immutable base = new immutable(Hash!(string, int))(data);
auto subject = base.insert("bar", 2).insert("foo", 3);

V[] values;
K[] keys;

foreach(K key, V value; subject) {
    values[values.length++] = value;
    keys[keys.length++] = key;
}

assert(values.length == 2);
assert(keys.length == 2);