Function interleave

src/CollectionImpl.js:638–646

(/*...collections*/)

  },

  interleave(/*...collections*/) {
    const collections = [this].concat(arrCopy(arguments));
    const zipped = zipWithFactory(this.toSeq(), IndexedSeq.of, collections);
    const interleaved = zipped.flatten(true);
    if (zipped.size) {
      interleaved.size = zipped.size * collections.length;
    }
    return reify(this, interleaved);
  },

  keySeq() {
    return Range(0, this.size);