Function mandel(xi As Double, yi As Double)

maxiter = 256
x = 0
y = 0

For i = 1 To maxiter
    If ((x * x) + (y * y)) > 4 Then Exit For
    xt = xi + ((x * x) - (y * y))
    y = yi + (2 * x * y)
    x = xt
    Next

mandel = i
End Function