src/main/java/org/organicdesign/fp/collections/PersistentVector.java
@Override public PersistentVector<E> concat(Iterable<? extends E> items) {
        return (PersistentVector<E>) ImList.super.concat(items);
    }
@Override public PersistentVector<E> concat(Iterable<? extends E> items) {
        return (PersistentVector<E>) ImList.super.concat(items);
    }
Efficiently adds items to the end of this PersistentVector.
@param items the values to insert
@return a new PersistentVector with the additional items at the end.