[mercury-users] What mode is necessary?

[Mike Potanin]

 Hi!
I want to make pred which receives on an input list_skel(free)
and ground on an output. Like satisfy_request from manual.
I make code:
:-inst listskel(Inst) == bound( [] ; [Inst | listskel(Inst)] ).
:-pred preffix(list(T), list(T)).
:-mode preffix(listskel(free)>>listskel(ground), in) is semidet.
%:-mode preffix(listskel(free)>>ground, in) is semidet.

and want use preffix([X,Y], [1,2,3,4]) where X, Y is free.
But this code do not compile.
What mode is necessary for first argument of preffix?
mmc --infer-modes do not infered it.

Similar code on Curry work:

preffix [] _ = success
preffix (x:t) (y:t1) | x =:= y = preffix t t1

goal | preffix [x,y] [1,2,3,4] = (x,y)
  where
    x free
    y free

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