function mul_inv(integer a, n)
    if n<0 then n = -n end if
    if a<0 then a = n - mod(-a,n) end if
    integer t = 0,  nt = 1,
            r = n,  nr = a;
    while nr!=0 do
        integer q = floor(r/nr)
        {t, nt} = {nt, t-q*nt}
        {r, nr} = {nr, r-q*nr}
    end while
    if r>1 then return "a is not invertible" end if
    if t<0 then t += n end if
    return t
end function

?mul_inv(42,2017)
?mul_inv(40, 1)
?mul_inv(52, -217)  /* Pari semantics for negative modulus */
?mul_inv(-486, 217)
?mul_inv(40, 2018)