concat(other) {
        let seq = this._iterable;
        return Seq.fromGenerator(function*() {
            yield* seq;
            yield* other;
        });
    }
Returns a sequence that iterates the receiving sequence's items and then the given iterable's items.
@param {*|!(A[])|!Seq.<A>} other
@returns {!Seq.<T|A>}
@template T, A