code: def unmap_namespace(self, plain_target_ns):
          
          # Return the same namespace if there are no included namespaces.
          if not self._regex_map and not self._plain:
              return plain_target_ns
  
          src_name_set = self._reverse_plain.get(plain_target_ns)
          if src_name_set:
              # Return the first (and only) item in the set
              for src_name in src_name_set:
                  return src_name
          # The target namespace could also exist in the wildcard namespaces
          for _, namespace in self._regex_map:
              original_name = match_replace_regex(
                  namespace_to_regex(namespace.dest_name),
                  plain_target_ns,
                  namespace.source_name,
              )
              if original_name:
                  return original_name
          return None
docs: Given a plain target namespace, return the corresponding source
          namespace.