diff --git a/autobrake_test.p8 b/autobrake_test.p8
index a000557..bf88058 100644
--- a/autobrake_test.p8
+++ b/autobrake_test.p8
@@ -128,15 +128,20 @@ function ship:draw()
  end
 end
 
+function calc_velocity(v0, t, vmax, drag)
+ local v1 = v0 + t
+ local sg = sgn(v1)
+ v1 -= sg*drag
+ if (sgn(v1) != sg) return 0
+ if (abs(v1) > vmax) return sg*vmax
+ return v1
+end
+
 function ship:update()
  local t = btn(0) and -1 or btn(1) and 1 or 0
  t *= self.thrust
  t = constraints:constrain(self, t)
- local s,lim=self.dx+t,self.maxspd
- local sg = sgn(s)
- s -= sg*self.drag
- if (sgn(s) != sg) s=0
- if (abs(s)>lim) s=sg*lim
+ local s = calc_velocity(self.dx, t, self.maxspd, self.drag)
 
  self.x += s
  self.dx = s
@@ -153,9 +158,11 @@ end
 constraints = {
  ymin=20,
  ymax=52,
+ color=10
 }
 
 function constraints:constrain(s, want)
+ self.color=10
  return want
 end
 
@@ -185,7 +192,7 @@ end
 
 function constraints:draw()
  if (not self.xmin) return
- rect(self.xmin, self.ymin, self.xmax, self.ymax, 10)
+ rect(self.xmin, self.ymin, self.xmax, self.ymax, self.color)
 end
 
 -->8