Fast-forwarding deep trees is problematic

[mbolis]

this program makes ffwd behave strangely:

var x = 0
var lato = 0.75
var angolo = 2
while (x < 20000) {
  forward(lato)
  right((x % 4 - 1.5) * angolo)
  x = x + 1
  angolo = angolo - 0.0005 * x * x * x
  color = hsl(360 / 20000 * x, 1, 0.5)
}