diff

Computes the difference, ``(d[i] = x[i+k] - x[i]) for i = 0 until x.size - k`` This is the discrete difference operator. For example, if k = 1, then ``d[0] = x[1] - x[0], d[1] = x[2] - x[1], ..., d[x.size - 2] = x[x.size -1 ]- x[x.size -2]`` and returns the new array of differences.