docs: def _get_rel_of_type(self, reltype):
          """
          Return single relationship of type *reltype* from the collection.
          Raises |KeyError| if no matching relationship is found. Raises
          |ValueError| if more than one matching relationship is found.
          """
          matching = [rel for rel in self.values() if rel.reltype == reltype]
          if len(matching) == 0:
              tmpl = "no relationship of type '%s' in collection"
              raise KeyError(tmpl % reltype)
          if len(matching) > 1:
              tmpl = "multiple relationships of type '%s' in collection"
              raise ValueError(tmpl % reltype)
          return matching[0]