public Expression negate() {
		if (getOperatorIndex() == LOG_AND_OP || getOperatorIndex() == LOG_OR_OP) {
			setOperatorIndex(getOperatorIndex() ^ 1);
			for (int i = 0; i < 2; i++) {
				subExpressions[i] = subExpressions[i].negate();
				subExpressions[i].parent = this;
			}
			return this;
		}
		return super.negate();
	}