diff --git a/extra/pong/pong.factor b/extra/pong/pong.factor
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+
+USING: kernel accessors locals math math.intervals math.order
+       namespaces sequences threads
+       ui
+       ui.gadgets
+       ui.gestures
+       ui.render
+       calendar
+       multi-methods
+       multi-method-syntax
+       combinators.short-circuit.smart
+       combinators.cleave.enhanced
+       processing.shapes
+       flatland ;
+
+IN: pong
+
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+: clamp-to-interval ( x interval -- x )
+  [ from>> first max ] [ to>> first min ] bi ;
+
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+TUPLE: <play-field> < <rectangle>    ;
+TUPLE: <paddle>     < <rectangle>    ;
+
+TUPLE: <computer>   < <paddle> { speed initial: 10 } ;
+
+: computer-move-left  ( computer -- ) dup speed>> move-left-by  ;
+: computer-move-right ( computer -- ) dup speed>> move-right-by ;
+
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+TUPLE: <ball> < <vel>
+  { diameter   initial: 20   }
+  { bounciness initial:  1.2 }
+  { max-speed  initial: 10   } ;
+
+: above-lower-bound? ( ball field -- ? ) bottom 50 - above? ;
+: below-upper-bound? ( ball field -- ? ) top    50 + below? ;
+
+: in-bounds? ( ball field -- ? )
+  {
+    [ above-lower-bound? ]
+    [ below-upper-bound? ]
+  } && ;
+
+:: bounce-change-vertical-velocity ( BALL -- )
+
+  BALL vel>> y neg
+  BALL bounciness>> *
+
+  BALL max-speed>> min
+
+  BALL vel>> (y!) ;
+
+:: bounce-off-paddle ( BALL PADDLE -- )
+
+   BALL bounce-change-vertical-velocity
+
+   BALL x   PADDLE center x   -   0.25 *   BALL vel>> (x!)
+
+   PADDLE top   BALL pos>> (y!) ;
+
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+: mouse-x ( -- x ) hand-loc get first ;
+
+:: valid-paddle-interval ( PADDLE PLAY-FIELD -- interval )
+    
+   PLAY-FIELD [ left ] [ right ] bi PADDLE width - [a,b] ;
+
+:: align-paddle-with-mouse ( PADDLE PLAY-FIELD -- )
+
+   mouse-x
+
+   PADDLE PLAY-FIELD valid-paddle-interval
+
+   clamp-to-interval
+
+   PADDLE pos>> (x!) ;
+   
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+! Protocol for drawing PONG objects
+
+GENERIC: draw ( obj -- )
+
+METHOD: draw ( <paddle> -- ) [ bottom-left ] [ dim>>          ] bi rectangle ;
+METHOD: draw ( <ball>   -- ) [ pos>>       ] [ diameter>> 2 / ] bi circle    ;
+
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+USE: syntax ! Switch back to core 'TUPLE:' instead of the one provided
+            ! by multi-methods
+
+TUPLE: <pong> < gadget draw closed ;
+
+M: <pong> pref-dim*    ( <pong> -- dim ) drop { 400 400 } ;
+M: <pong> draw-gadget* ( <pong> --     ) draw>> call      ;
+M: <pong> ungraft*     ( <pong> --     ) t >>closed drop  ;
+
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+: make-draw-closure ( -- closure )
+
+  ! Establish some bindings
+
+  [let | PLAY-FIELD [ T{ <play-field> { pos {  0  0 } } { dim { 400 400 } } } ]
+         BALL       [ T{ <ball>       { pos { 50 50 } } { vel {   3   4 } } } ]
+
+         PLAYER   [ T{ <paddle>   { pos { 200 396 } } { dim { 75 4 } } } ]
+         COMPUTER [ T{ <computer> { pos { 200   0 } } { dim { 75 4 } } } ] |
+
+    ! Define some internal words in terms of those bindings ...
+
+    [wlet | align-player-with-mouse [ ( -- )
+              PLAYER PLAY-FIELD align-paddle-with-mouse ]
+
+            move-ball [ ( -- ) BALL 1 move-for ]
+
+            player-blocked-ball? [ ( -- ? )
+              BALL PLAYER { [ above? ] [ in-between-horizontally? ] } && ]
+
+            computer-blocked-ball? [ ( -- ? )
+              BALL COMPUTER { [ below? ] [ in-between-horizontally? ] } && ]
+
+            bounce-off-wall? [ ( -- ? )
+              BALL PLAY-FIELD in-between-horizontally? not ] |
+
+      ! Note, we're returning a quotation.
+      ! The quotation closes over the bindings established by the 'let'.
+      ! Thus the name of the word 'make-draw-closure'.
+      ! This closure is intended to be placed in the 'draw' slot of a
+      ! <pong> gadget.
+      
+      [
+
+        BALL PLAY-FIELD in-bounds?
+          [
+            align-player-with-mouse
+              
+            move-ball
+  
+            ! computer reaction
+  
+            BALL COMPUTER to-the-left-of?  [ COMPUTER computer-move-left  ] when
+            BALL COMPUTER to-the-right-of? [ COMPUTER computer-move-right ] when
+
+            ! check if ball bounced off something
+              
+            player-blocked-ball?   [ BALL PLAYER   bounce-off-paddle  ] when
+            computer-blocked-ball? [ BALL COMPUTER bounce-off-paddle  ] when
+            bounce-off-wall?       [ BALL reverse-horizontal-velocity ] when
+
+            ! draw the objects
+              
+            COMPUTER draw
+            PLAYER   draw
+            BALL     draw
+  
+          ]
+        when
+
+      ] ] ] ( -- closure ) ; ! The trailing stack effect here is a workaround.
+                             ! The stack effects in the wlet expression throw
+                             ! off the effect for the whole word, so we reset
+                             ! it to the correct one here.
+
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+:: pong-loop-step ( PONG -- ? )
+  PONG closed>>
+    [ f ]
+    [ PONG relayout-1 25 milliseconds sleep t ]
+  if ;
+
+:: start-pong-thread ( PONG -- ) [ [ PONG pong-loop-step ] loop ] in-thread ;
+
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+: play-pong ( -- )
+
+  <pong> new-gadget
+    make-draw-closure >>draw
+  dup "PONG" open-window
+    
+  start-pong-thread ;
+
+! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+: play-pong-main ( -- ) [ play-pong ] with-ui ;
+
+MAIN: play-pong-main
\ No newline at end of file