get "libhdr"

let mulinv(a, b) =
    b<0 -> mulinv(a, -b),
    a<0 -> mulinv(b - (-a rem b), b),
    valof
    $(  let t, nt, r, nr = 0, 1, b, a rem b
        until nr = 0
        $(  let tmp, q = ?, r / nr
            tmp := nt ; nt := t - q*nt ; t := tmp
            tmp := nr ; nr := r - q*nr ; r := tmp
        $)
        resultis r>1 -> -1,
                 t<0 -> t + b,
                 t
    $)

let show(a, b) be
$(  let mi = mulinv(a, b)
    test mi>=0
        do writef("%N, %N -> %N*N", a, b, mi)
        or writef("%N, %N -> no inverse*N", a, b)
$)

let start() be
$(  show(42, 2017)
    show(40, 1)
    show(52, -217)
    show(-486, 217)
    show(40, 2018)
$)