private void check (subExpression x) throws IllegalLineException {
		if ((x.name.contains("(")) || (x.name.contains(")"))) {
			throw new IllegalLineException("parentheses in proposition");
		}
		
		if (x.myLeft != null) {
			if ((x.myRight == null) && (x.name != "~")) {
				throw new IllegalLineException("unbalanced expression");
			}
			check(x.myLeft);
		}
		if (x.myRight != null) {
			if (x.myLeft == null) {
				throw new IllegalLineException("unbalanced expression");
			}
			check(x.myRight);
		}
	}