def add_signer(*keys)
      keys.each do |key|
        err = GPGME::gpgme_signers_add(self, key)
        exc = GPGME::error_to_exception(err)
        raise exc if exc
      end
    end
def add_signer(*keys)
      keys.each do |key|
        err = GPGME::gpgme_signers_add(self, key)
        exc = GPGME::error_to_exception(err)
        raise exc if exc
      end
    end
Add _keys_ to the list of signers.