IsUsEin
Determines whether a given input is a valid US Employer Identification Number (EIN).
This class extends the Function class and provides a method to check if a given input is a valid US Employer Identification Number (EIN). The implementation follows the specified validation rules.
The IsUsEin class is meant to be instantiated and used to validate US Employer Identification Number (EIN). It inherits common functionality from the Function base class.