anchor.diff:  			(:li "Up and down arrows to tilt")
   			(:li "Right and left arrows to rotate")))))
         (:div :id "nav"
  -	    (:button :id "pause" :name "pause" "Pause")
  +	    (:button :id "pause" "Pause")
  +	    (:button :id "reset" "Reset")
   	    (:p (:b (:a :href "http://www.patreon.com/bld" "Next: Help develop it at Patreon")))
   	    (:p (:b (:a :href "/reflect.html" "Previous: Sunlight reflecting off of a solar sail")))
   	    (:p (:b (:a :href "/index.html" "Home"))))
  
positive.code: 	    (:h1 "Force from Absorbed Sunlight on a Solar Sail")
  	    (:ul
  	     (:li "Sunlight pushes a sail in the direction it shines. The force is proportional to the light absorbed.")
  	     (:li (:b "Challenge:") "At what sun incidence angles is the acceleration maximum, half, and zero?"))
  	    (:table
  	     :id "tilt-controls"
  	     (:tr (:td (:b "Sun incidence")) (:td :id "incidence" "0") (:td "deg"))
  
negative.code:       (:div :id "help"
  	    (:h2 (:a :href "#" :onclick "toggleDiv(\"helpText\"); return false;" "Help"))
  	    (:div :id "helpText"
  		  (:li "The sail resets after travelling 100 m.")
  		  (:li "Rotate the view by clicking or touching and dragging.")
  		  (:li "To zoom:"
  		       (:ul
  