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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 signCertAs(self, cert, signas):
        '''
        Signs a certificate with a CA keypair.

        Args:
            cert (OpenSSL.crypto.X509): The certificate to sign.
            signas (str): The CA keypair name to sign the new keypair with.

        Examples:
            Sign a certificate with the CA "myca":

                cdir.signCertAs(mycert, 'myca')

        Returns:
            None
        '''
        cakey = self.getCaKey(signas)
        if cakey is None:
            raise s_exc.NoCertKey('Missing .key for %s' % signas)
        cacert = self.getCaCert(signas)
        if cacert is None:
            raise s_exc.NoCertKey('Missing .crt for %s' % signas)

        cert.set_issuer(cacert.get_subject())
        cert.sign(cakey, self.signing_digest)