Further library additions and REPL updates

src/Research.hs

@@ -78,7 +78,7 @@ ofNumber _ = Left "Invalid Tree Calculus number"
 ofString :: T -> Either String String
 ofString tc = case ofList tc of
   Right list -> traverse (fmap toEnum . ofNumber) list
-  Left err -> Left err
+  Left err -> Left "Invalid Tree Calculus string"
 
 ofList :: T -> Either String [T]
 ofList Leaf = Right []