code: def sign(self, entry, signer=None):
          
          if (self.get(entry) is not None):
              return
          if (entry.rrsig is None) and (self.private is not None):
              entry.rrsig = DNSSignatureS(entry.name,
                      _TYPE_RRSIG, _CLASS_IN, entry, self.private, signer)
          self.add(entry)
          if (self.private is not None):
              self.add(entry.rrsig)
docs: Adds and sign an entry