/* Take the nth root of 'a' (a positive real number).
 * 'n' must be an integer.
 * Result will have 'd' digits after the decimal point.
 */
define r(a, n, d) {
    auto e, o, x, y, z

    if (n == 0) return(1)
    if (a == 0) return(0)

    o = scale
    scale = d
    e = 1 / 10 ^ d

    if (n < 0) {
        n = -n
        a = 1 / a
    }

    x = 1
    while (1) {
        y = ((n - 1) * x + a / x ^ (n - 1)) / n
        z = x - y
        if (z < 0) z = -z
        if (z < e) break
        x = y
    }
    scale = o
    return(y)
}