src/main/java/org/organicdesign/fp/xform/Xform.java
@Override public Xform<A> precat(Iterable<? extends A> list) {
        if (list == null) { throw new IllegalArgumentException("Can't precat a null iterable"); }
        return new AppendIterDesc<>(of(list), this);
    }
@Override public Xform<A> precat(Iterable<? extends A> list) {
        if (list == null) { throw new IllegalArgumentException("Can't precat a null iterable"); }
        return new AppendIterDesc<>(of(list), this);
    }
}