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From: rjmh@glasgow.glasgow.UUCP (Prof. John Hughes)
Newsgroups: net.lang.prolog
Subject: Re: miscellany re income tax planning system
Message-ID: <578@glasgow.glasgow.UUCP>
Date: Mon, 2-Jun-86 16:33:31 EDT
Article-I.D.: glasgow.578
Posted: Mon Jun  2 16:33:31 1986
Date-Received: Thu, 5-Jun-86 07:32:08 EDT
References: <1219@lsuc.UUCP> <164@cui.UUCP>
Reply-To: rjmh@glasgow.UUCP (Prof. John Hughes)
Organization: Comp Sci Dept, Glasgow Univ, Scotland
Lines: 19


>> From: dave@lsuc.UUCP
>> Subject: miscellany re income tax planning system
>> ...
>> Third, I'm currently wrestling with the task of generating,
>> for a list, every list which is a subset of that list. Thus,
>> for [a,b,c,d], I want findall to be able to find each of
>> [a,b] [a,c] [a,d] [a,b,c] [a,b,d] [a,c,d] [b,c] [b,d] [b,c,d] [c,d]. 
>

This should do the trick:

included(Subset,Set) is true if Subset is a subset of Set

included([],Set).
included([X|Subset],Set):-append(_,[X|Rest],Set),included(Subset,Rest).

It only includes subsets whose elements are in the same order as in the
original list.