src/main/java/org/organicdesign/fp/collections/PersistentVector.java
@SuppressWarnings("unchecked")
    public static <T> MutableVector<T> emptyMutable() {
        PersistentVector<T> e = empty();
        return e.mutable();
    }
@SuppressWarnings("unchecked")
    public static <T> MutableVector<T> emptyMutable() {
        PersistentVector<T> e = empty();
        return e.mutable();
    }
Returns a new mutable vector.  For some reason calling empty().mutable() sometimes requires
an explicit type parameter in Java, so this convenience method works around that.