Rolf Ade boasted a question to the chat asking how to form the Cartesian product of a set of lists. That is, given a list like, { { a b c } { d e f } } he wanted {{a d} {a e} {a f} {b d} {b e} {b f} {c d} {c e} {c f}} He also wanted it to generalize to higher dimension: given {{a b} {c d} {e f}} he wanted {{a c e} {a c f} {a d e} {a d f} {b c e} {b c f} {b d e} {b d f}} and so on. [Kevin Kenny] proposed the following: proc crossProduct { listOfLists } { if { [llength \$listOfLists] == 0 } { return [list [list]] } else { set result [list] foreach elt [lindex \$listOfLists 0] { foreach combination [crossProduct [lrange \$listOfLists 1 end]] { lappend result [linsert \$combination 0 \$elt] } } return \$result } } puts [crossProduct {{a b c} {d e f} {g h i}}] This solution is by no means the fastest available, but it appears to work for the purpose. ---- [Arjen Markus] An interesting variation on this theme: how to generate the set of subsets containing 1, 2, 3 ... elements. For example: {a b c d e} will give rise to: {{a} {b} {c} {d} {e}} {{a b} {a c} {a d} {a e} {b c} {b d} {b e} {c d} {c e} {d e}} ... It does not seem quite trivial. The answer is posted in [Power set of a list]. ---- [Category Mathematics]