flatMap(sequenceProjection) {
        let seq = this._iterable;
        return Seq.fromGenerator(function*() {
            for (let e of seq) {
                yield* sequenceProjection(e);
            }
        });
    }
Returns a sequence iterating the concatenated results of applying an iterable-returning transformation to the
items of the receiving sequence.
@param {!function(T): !Iterable<R>} sequenceProjection
@returns {!Seq.<T>}
@template T, R