index.js
function grayInverse (g) { // : Int -> Int
    var m = bitwise.bitPrecision(g), inverse = g, j = 1
    while (j < m) {
        inverse = inverse ^ (g >> j)
        j++
    }
    return inverse
}
function grayInverse (g) { // : Int -> Int
    var m = bitwise.bitPrecision(g), inverse = g, j = 1
    while (j < m) {
        inverse = inverse ^ (g >> j)
        j++
    }
    return inverse
}
returns a non-negative int 'inverse' such that graycode(inverse) = g