private void checkVariableName(String varName) throws UnparsableExpressionException {
		char[] name = varName.toCharArray();
		for (int i = 0; i < name.length; i++) {
			if (i == 0) {
				if (!Character.isLetter(name[i]) && name[i] != '_') {
					throw new UnparsableExpressionException(varName + " is not a valid variable name: character '" + name[i] + " at " + i);
				}
			} else {
				if (!Character.isLetter(name[i]) && !Character.isDigit(name[i]) && name[i] != '_') {
					throw new UnparsableExpressionException(varName + " is not a valid variable name: character '" + name[i] + " at " + i);
				}
			}
		}
	}