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