Fix for GHC 9.6

[vvs-]

Just for those who failed to build this on recent GHC. The fix was suggested by AI. I've applied it and it seems to work:

diff --git a/source/Main.hs b/source/Main.hs
index 0feef28..0416207 100644
--- a/source/Main.hs
+++ b/source/Main.hs
@@ -1,5 +1,6 @@
 module Main where
 
+import           Control.Monad
 import           Control.Monad.Trans
 import           Control.Monad.State
 import           Control.Exception
diff --git a/source/Stlc/Gentzen.hs b/source/Stlc/Gentzen.hs
index ae5c7e5..3d82adb 100644
--- a/source/Stlc/Gentzen.hs
+++ b/source/Stlc/Gentzen.hs
@@ -1,3 +1,5 @@
+{-# LANGUAGE QuantifiedConstraints #-}
+
 module Stlc.Gentzen
   ( gentzendiagram
   , showProofTree
@@ -11,9 +13,12 @@ import NamedLambda
 import Data.Bifunctor
 import qualified Data.Map as Map
 
-
 data ProofTree a l = Inference a | Deduction a l [ProofTree a l]
 
+instance Functor (ProofTree a) where
+  fmap _ (Inference a)      = Inference a
+  fmap g (Deduction a l ps) = Deduction a (g l) (fmap (fmap g) ps)
+
 instance Bifunctor ProofTree where
   bimap f _ (Inference a)      = Inference (f a)
   bimap f g (Deduction a l ps) = Deduction (f a) (g l) (map (bimap f g) ps)