code: def _add_plain_namespace(self, namespace):
          
          src_name = namespace.source_name
          target_name = namespace.dest_name
          src_names = self._reverse_plain.setdefault(target_name, set())
          src_names.add(src_name)
          if len(src_names) > 1:
              # Another source namespace is already mapped to this target
              existing_src = (src_names - set([src_name])).pop()
              raise errors.InvalidConfiguration(
                  "Multiple namespaces cannot be combined into one target "
                  "namespace. Trying to map '%s' to '%s' but there already "
                  "exists a mapping from '%s' to '%s'"
                  % (src_name, target_name, existing_src, target_name)
              )
  
          self._plain[src_name] = namespace
          src_db, _ = src_name.split(".", 1)
          target_db, _ = target_name.split(".", 1)
          self._plain_db.setdefault(src_db, set()).add(target_db)
docs: Add an included and possibly renamed non-wildcard Namespace.