Slightly tangential, but here's my own Sierpinski triangle program from a couple of decades ago, which IMO is considerably simpler than what they show: %!

Slightly tangential, but here's my own Sierpinski triangle program from a couple of decades ago, which IMO is considerably simpler than what they show: %! /threshold 4 def /Sierpinski { dup threshold ge { 3 {dup 2 div Sierpinski dup 0 rmoveto 120 rotate} repeat } { 3 {dup 0 rlineto 120 rotate} repeat closepath } ifelse pop } bind def 50 50 moveto 512 Sierpinski fill showpage It actually can be simplified a bit more, and the explicit recursion can be removed entirely, since there's already a free operand stack.