# comb([qw(a b c)],[qw(1 2 3)],[qw(- + *)]);
# -> [[a,1,-],[a,1,+],[a,1,*],
#     [a,2,-],[a,2,+],[a,2,*],
#     [a,3,-],[a,3,+],[a,3,*],
#     [b,1,-],[b,1,+],[b,1,*],
#     [b,2,-],[b,2,+],[b,2,*],
#     [b,3,-],[b,3,+],[b,3,*],
#     [c,1,-],[c,1,+],[c,1,*],
#     [c,2,-],[c,2,+],[c,2,*],
#     [c,3,-],[c,3,+],[c,3,*]]
#