code: def get_custom_implementations(self):
          
          for trname in self.custom_implems:
              attr = self.transitions_at[trname]
              implem = self.implementations[trname]
              yield (trname, attr, implem)
docs: Retrieve a list of cutom implementations.
  
          Yields:
              (str, str, ImplementationProperty) tuples: The name of the attribute
                  an implementation lives at, the name of the related transition,
                  and the related implementation.