private Tree string(Tree t, String s, int begin, int end) throws ParseException {
		char quote = s.charAt(begin);
		if (t.node.equals("CharacterLiteral") && quote != '\'') {
			return null;
		}
		if (t.node.equals("StringLiteral") && quote != '"') {
			return null;
		}
		for (int i = begin + 1; i < end; i++) {
			char ch = s.charAt(i);
			if (ch == '\\') {
				i++;
			} else if (ch == '\n') {
				throw new ParseException("String constant exceeds line at " + pos(i));
			} else if (ch == quote) {
				return new Tree(s, begin, i + 1, t);
			}
		}
		return null;
	}