> perl pairs.pl e 2
15 pairings:
( (a,b), (c,d) )
( (a,b), (c,e) )
( (a,b), (d,e) )
( (a,c), (b,d) )
( (a,c), (b,e) )
( (a,c), (d,e) )
( (a,d), (b,c) )
( (a,d), (b,e) )
( (a,d), (c,e) )
( (a,e), (b,c) )
( (a,e), (b,d) )
( (a,e), (c,d) )
( (b,c), (d,e) )
( (b,d), (c,e) )
( (b,e), (c,d) )