private String moveCardOntoKingPile(List<Card> from, int indexFrom, int indexTo) {
        List<Card> toPile = kingPiles.get(indexTo);
        if (indexFrom == -1) indexFrom = from.size() - 1;

        Card toMove = from.get(indexFrom);
        //Assumes at least one card remaining.
        Card pileTop = toPile.get(toPile.size() - 1);
        if (toMove.getSuit().ordinal() == pileTop.getSuit().ordinal()) {
            if (toMove.getRank().ordinal() == pileTop.getRank().ordinal() - 1) {
                toPile.add(from.remove(indexFrom));
                return "";
            } else {
                return toMove.getRank().name() + " is not one lower than " + pileTop.getRank().name();
            }

        } else {
            return "Can only move to a pile of the same suit";
        }

    }