def set_principal_credit_string(self, credit_string=None):
pass<EOL>
Sets the principal credit string.

        arg:    credit_string (string): the new credit string
        raise:  InvalidArgument - ``credit_string`` is invalid
        raise:  NoAccess - ``Metadata.isReadOnly()`` is ``true``
        raise:  NullArgument - ``credit_string`` is ``null``
        *compliance: mandatory -- This method must be implemented.*