2014-05-06: polish.

polish.

diff --git a/examples/d20world.w b/examples/d20world.w
--- a/examples/d20world.w
+++ b/examples/d20world.w
@@ -200,10 +200,6 @@ format #t "Diffuse ~A\n" 0.01
 d20-diffuse world neighbors 0.01
 display : d20-as-text world
 newline
-format #t "Advect ~A\n" 0.1
-d20-advect world advection-directions 0.1
-display : d20-as-text world
-newline
 format #t "Diffuse ~A\n" 0.1
 d20-diffuse world neighbors 0.1
 display : d20-as-text world