@Override
	public Expression customize(String funcName) {
		try {
			Class<Function> found = static_mappings.get(funcName);
			if (found != null) {
				return found.newInstance();
			}
			found = mappings.get(funcName);
			if (found != null) {
				return found.newInstance();
			}
			if (parent != null) {
				return parent.customize(funcName);
			}
			throw new FormulaException("Can not find function :" + funcName);
		} catch (Exception ex) {
			throw new FormulaException("Can not create function :" + funcName);
		}
	}