sum() {
        let total = 0;
        let first = true;
        for (let e of this._iterable) {
            total = first ? e : total + e;
            first = false;
        }
        return total;
    }
Adds up the numbers in the sequence, using the `+` operator, and returns the total.
The empty sum is defined to be 0, to satisfy the invariant that X.concat([s]).sum() === X.sum() + s.
@returns {T|!number|*}
@template T