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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def find_signature(self, signature_id=None, signer_email_address=None):
        ''' Return a signature for the given parameters

            Args:

                signature_id (str):             Id of the signature to retrieve.
                signer_email_address (str):     Email address of the associated signer for the signature to retrieve.

            Returns:
                A Signature object or None

        '''
        if self.signatures:
            for signature in self.signatures:
                if signature.signature_id == signature_id or signature.signer_email_address == signer_email_address: 
                    return signature