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mba:combinatorial_sets johnz$ swipl
Welcome to SWI-Prolog (threaded, 64 bits, version 8.0.3)
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software.
Please run ?- license. for legal details.

For online help and background, visit http://www.swi-prolog.org
For built-in help, use ?- help(Topic). or ?- apropos(Word).

?- consult('combinatorial_sets.pro').
true.

?- permutations(a,b).
ab ba 
true.

?- permutations(a,b,c).
abc acb bac bca cab cba 
true.

?- permutations(a,b,c,d).
abcd abdc acbd acdb adbc adcb bacd badc bcad bcda bdac bdca cabd cadb cbad cbda cdab cdba dabc dacb dbac dbca dcab dcba 
true.

?- permutations(a,b,c,d,e).
abcde abced abdce abdec abecd abedc acbde acbed acdbe acdeb acebd acedb adbce adbec adcbe adceb adebc adecb aebcd aebdc aecbd aecdb aedbc aedcb bacde baced badce badec baecd baedc bcade bcaed bcdae bcdea bcead bceda bdace bdaec bdcae bdcea bdeac bdeca beacd beadc becad becda bedac bedca cbade cbaed cbdae cbdea cbead cbeda cabde cabed cadbe cadeb caebd caedb cdbae cdbea cdabe cdaeb cdeba cdeab cebad cebda ceabd ceadb cedba cedab dbcae dbcea dbace dbaec dbeca dbeac dcbae dcbea dcabe dcaeb dceba dceab dabce dabec dacbe daceb daebc daecb debca debac decba decab deabc deacb ebcda ebcad ebdca ebdac ebacd ebadc ecbda ecbad ecdba ecdab ecabd ecadb edbca edbac edcba edcab edabc edacb eabcd eabdc eacbd eacdb eadbc eadcb 
true.

?- perm(s(a,b),p(A,B)).
A = a,
B = b ;
A = b,
B = a.

?- comb(set(a,b,c,d),comb(A,B),extras(_,_)).
A = a,
B = b ;
A = a,
B = c ;
A = a,
B = d ;
A = b,
B = c ;
A = b,
B = d ;
A = c,
B = d.

?- comb(set(a,b,c,d),comb(A,B),extras(_)).
false.

?- halt.
mba:combinatorial_sets johnz$