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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 sender(self, body, stamp, url, sig):
 """Validate request is from Alexa. Verifying that the Request was Sent by Alexa: https://goo.gl/AcrzB5. Checking the Signature of the Request: https://goo.gl/FDkjBN. Checking the Timestamp of the Request: https://goo.gl/Z5JhqZ Args: body: str. HTTPS request body. stamp: str. Value of timestamp within request object of HTTPS request body. url: str. SignatureCertChainUrl header value sent by request. sig: str. Signature header value sent by request. Returns: bool: True if valid, False otherwise. """

        if not timestamp(stamp):
            return False

        if self.url != url:
            if not signature_cert_chain_url(url):
                return False

            certs = retrieve(url)
            if not certs:
                return False

            if not cert_chain(certs):
                return False

            self.url = url
            self.cert = certs[0]

        if not signature(self.cert, sig, body):
            return False

        return True