world docs updated

svn: r2518

teachpack/htdp/Docs/world.thtml

@@ -33,6 +33,27 @@ students to simulate a small world of animated drawings and games:
    or the events of releasing a key on the keypad. 
 </pre>
 
+<li><code>{(idx on-mouse-event)} : (World Number Number MouseEvent ->
+   World) -> true</code><br> <code>(on-mouse-event clack)</code> means that
+   DrScheme must call <code>clack</code> on the current world, the current
+   <code>x</code> and <code>y</code> coordinates of the mouse, and and a
+   (representation of the) mouseevent for every action of the mouse the programmer
+   (user of the computer) makes; it uses the result as the next world
+
+<pre>
+<code>
+  ;; A MouseEventType is one of:
+  ;; - 'button-down
+  ;; - 'button-up
+  ;; - 'drag
+  ;; - 'move
+  ;; - 'enter
+  ;; - 'leave
+</code>
+   The symbols denote the appropriate action with the mouse and (any of)
+   its button(s).
+</pre>
+
 <li><code>{(idx on-redraw)} : (World -> Image) -> true</code><br>
    <code>(on-tick-event world->image)</code> means that DrScheme 
    calls <code>world->image</code> whenever the canvas must be redrawn; the
@@ -69,6 +90,8 @@ students to simulate a small world of animated drawings and games:
    shows the list of images in loi, time-delayed
 </menu></p>
 
-<p>Finally, the teachpack provides all the functions that image.ss provides. </p>
+<p>Finally, the teachpack provides all the functions that image.ss provides
+except <code>add-line</code>, which has a slightly different functionality.</p>
+
 
 {(include "foot.tinc")}