Hi, Sometimes I get a pattern match error, with output such as: pattern match failure: uniqueHite_v9935 (instEq_v280 (instEq_v20 instEq_v11) (instEq_v20 instEq_v11)) (map head [[]] ++ (map_v810 (map (uniqueHite_v9931 (instOrd_v21 instOrd_v12))) (mapMaybe (uncurry equalGiven) (map_v810 (IFunc_Func "map~1" [0] (IExpr_Case (IExpr_Var 0) [("[]",IExpr_Make "[]" []),(":",IExpr_Make ":" [IExpr_Apply (IExpr_Lambda [1] IExpr_Unknown) [IExpr_Sel (IExpr_Var 0) "hd"],IExpr_Call "map~2" [IExpr_Sel (IExpr_Var 0) "tl"]])]) [],) (_SEL (,) (select_v1958 (foldr (select (groupSetExtract_v7750 (Make.Eq (instEq_v20_v1493 instEq_v11) (Eq_v481 (Make.Eq (instEq_v20_v1493 instEq_v11) (Eq_v481 (Make.Eq (instEq_v20_v1493 instEq_v11) (Eq_v481 (Make.Eq (instEq_v20_v1493 instEq_v11) (Eq_v481 (Make.Eq (instEq_v20_v1493 instEq_v11) (Eq_v481 (Make.Eq (instEq_v20_v1493 instEq_v11) (Eq_v481 (Make.Eq (instEq_v20_v1493 instEq_v11) (Eq ... Where the output goes on forever. This is unfortunate, because having sped up the screen printing of WinHugs quite considerably, now its a race to hit the stop button before this stuff has taken up the whole screen buffer (100Kb) and pushed the name of the function causing the error off the top. I guess the cause of this error is dictionary transformations resulting in a nested dictionary? Is there any way to stop this happening? Is it a known bug - it happens reasonably regularly to me. Thanks Neil