MAKR 2009-04-09 a list zipper. This was briefly discussed on the chat today by RS, kruzalex, and teo. The implementation could be like:

    proc lzip {l1 l2} {
        set res {}
        foreach x $l1 y $l2 {
            lappend res $x $y
        set res

would lead to

    $ lzip {a b c} {1 2 3}
    a 1 b 2 c 3

A more generic form is available as [interleave].