c: {[h;t]{(h;t)x}}                  / cons
    h: {x 0}                            / head
    t: {x 1}                            / tail
    n: {h y t/x}                        / nth
    p: h'-1_(~^:)t\                     / print
    r: {{$[^y;x;c[h y;x]o t y]}[0N;x]}  / reverse