'Add the CA certificate to the list of preferred signers for this context.
The list of certificate authorities will be sent to the client when the
server requests a client certificate.
:param certificate_authority: certificate authority\'s X509 certificate.
:return: None'

def add_client_ca(self, certificate_authority):

    if (not isinstance(certificate_authority, X509)):
        raise TypeError('certificate_authority    must    be    an    X509    instance')
    add_result = _lib.SSL_CTX_add_client_CA(self._context, certificate_authority._x509)
    if (not add_result):
        _raise_current_error()
