DeORer does not correctly handle `not` literals

[gsingh93]

Given the GDL (<= p (not (or a b))), I'd expect this to de-or to (<= p (not a) (not b)), due to De Morgan's law, i.e., the negation of a disjunction is the conjunction of the negated terms in the disjunction. However, the DeORer class prints out ( <= p ( not a ) ) ( <= p ( not b ) ).

The following code is an example:

    public static void main(String[] args) {
        ArrayList<Gdl> l = new ArrayList<>();
        GdlConstant p = GdlPool.getConstant("p");
        GdlConstant a = GdlPool.getConstant("a");
        GdlConstant b = GdlPool.getConstant("b");

        l.add(GdlPool.getRule(GdlPool.getProposition(p),
                new GdlLiteral[] { GdlPool.getNot(GdlPool.getOr(
                        new GdlLiteral[] { GdlPool.getProposition(a), GdlPool.getProposition(b)})) }));

        List<Gdl> res = DeORer.run(l);
        for (Gdl g : res) {
            System.out.println(g.toString());
        }
    }

[gsingh93]

Here is my implementation of deorer in Rust: https://github.com/gsingh93/ggp-rs/blob/master/src/prover/deorer.rs

If I return a list of nots, I add all those nots to the end of the original rule, remove the original literal, and add that to the list.