Labeled effects.

An effect can be assigned multiple labels and you can have all of them available at the same time.

>>> import Effectful.Reader.Static

>>> :{
 action
   :: ( Labeled "a" (Reader String) :> es
      , Labeled "b" (Reader String) :> es
      , Reader String :> es
      )
   => Eff es String
 action = do
   a <- labeled @"b" @(Reader String) $ do
     labeled @"a" @(Reader String) $ do
       ask
   b <- labeled @"b" @(Reader String) $ do
     ask
   pure $ a ++ b
:}

>>> :{
 runPureEff @String
   . runLabeled @"a" (runReader "a")
   . runLabeled @"b" (runReader "b")
   . runReader "c"
   $ action
:}
"ab"