line-mode
read-nums
counter
even
f:
    54
    chunks
    2dup
    head
    6
    take-end
        over
        index
        inc
        drop
    b4
    each
    rot
    reverse
    over
    length
    drop
    rot
    extract-array
    swap
    reverse
        rot
            over
            index
            not
            drop
        b4
        each
    t4
    catenate #(list+list)
    length