1 files changed, 9 insertions, 1 deletions
diff --git a/Term.hs b/Term.hs
index 20c70ce..23fc5c5 100644
--- a/Term.hs
+++ b/Term.hs
@@ -1,5 +1,8 @@
 module Term (
     Term(..),
+    Substitution,
+
+    nullTerm,
 
     alphaEquiv,
     alphaConvert,
@@ -31,7 +34,8 @@ data Term = TVar { tVar :: Var }
           | TApp { tAppLeft :: Term
                  , tAppRight :: Term }
           | TAbs { tAbsVar :: Term
-                 , tAbsTerm :: Term } deriving (Ord)
+                 , tAbsTerm :: Term }
+          | TNull deriving (Ord)
 
 type Substitution = ( [(Name,Type)], [(Var,Term)] )
 
@@ -50,6 +54,10 @@ instance Eq Term where
 
 
 
+nullTerm :: Term
+nullTerm = TNull
+
+
 alphaEquiv :: Term -> Term -> Bool
 alphaEquiv a b =
     let equiv = \term1 term2 varmap1 varmap2 depth ->