Question

I want to make prolog returning me all possible sublists of given list so I wrote:

subSet(L,S):- append(_,L2,L),append(S,_,L2).

In that way I get result like:

Out = [] ;
Out = [a] ;
Out = [a, b] ;
Out = [a, b, c] ;
Out = [] ;
Out = [b] ;
Out = [b, c] ;
Out = [] ;
Out = [c] ;
Out = [] ;

What I have to do to get rid of repeating empty list?

Answer 1

Rule out that you are describing the empty list at all and add it manually once.

list_subseq(_,[]).
list_subseq(L, S) :- S = [_|_], ....

Nevertheless, expressing subsequences is often better done with DCGs. See the first definition in this response. Using append/3 might work, but leads to code that is quite difficult to read.