revived GADT generation

src/compiler/GF/Compile/PGFtoHaskell.hs

@@ -49,14 +49,15 @@ haskPreamble name =
  [
   "module " ++ name ++ " where",
   "",
-  "import PGF",
+  "import PGF hiding (Tree)",
+  "import qualified PGF",
   "----------------------------------------------------",
   "-- automatic translation from GF to Haskell",
   "----------------------------------------------------",
   "", 
   "class Gf a where",
-  "  gf :: a -> Tree",
-  "  fg :: Tree -> a",
+  "  gf :: a -> PGF.Tree",
+  "  fg :: PGF.Tree -> a",
   "",
   predefInst "GString" "String"  "unStr"    "mkStr",
   "",