Prolog - writing a combination of k numbers in the given list

I am trying to define a function in prolog that takes arguments of the form combination(3,[a,b,c,d],L) , the result returns

L=a,b,c
 L=a,b,d
 L=a,c,d
 L=b,c,d
 

My implementation is as follows:

combination(K,argList,L):-
 unknown(X,argList,Y),
 Z is select(X,argList),
 length(Z,K),
 L is Z,
 combination(K,Z,L).
 
 unknown(X,[X|L],L).
 unknown(X,[_|L],R) :- unknown(X,L,R).
 

The unknown predicate behaves as follows:
![enter image description here][1]

Please help.

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