🚀 go-pugleaf

From article <1992Oct29.140104.7751@coli.uni-sb.de>, by ulma@coli.uni-sb.de (Ulrich Mayring):
> Hello netters, I have a problem:
> 
> I have a list and need to recursively pick two elements out of it. I need
> every pair of elements in every order, however the order of the pair itself
> is not important. It should also be possible to pick an element and the []
> as a pair, thus guaranteeing that in the end I will have every pair and
> every single element. Then I will test the pair (or single) and if it doesn't
> suit my purpose I want to go back into the recursion to get the next one.
> 
> 
> Thanx a lot if you can help,
> 
> 
>      Ulrich Mayring
> 
>        - Also wir waren mit grau eigentlich sehr zufrieden -     


% no substitute for simplicity, i always say...

twins(L,X) :- 	subset(L,X), (X = [_,_] | X = [_] | X = []).

subset([],[]).
subset([H|T],[H|REST]) :- subset(T,REST).
subset([H|T],REST) :- subset(T,REST).   
                                          
% twins([a,b,c],Twins)?
%
% Twins = [a, b]
% 
% Twins = [a, c]
% 
% Twins = [a]
% 
% Twins = [b, c]
%
% Twins = [b]
% 
% Twins = [c]
% 
% Twins = []

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