diff --git a/scribblings/cs19.scrbl b/scribblings/cs19.scrbl
index e7f1ece..799965f 100644
--- a/scribblings/cs19.scrbl
+++ b/scribblings/cs19.scrbl
@@ -79,6 +79,10 @@ functions.  One difference introduced by the web is the web page
 itself: because the page itself is a source of state, it too will be
 passed to callbacks.
 
+The world-updating callbacks may optionally take an @tech{event} object, which 
+provides additional information about the event that triggered the callback.
+
+
 This library presents a functional version of the DOM in the form of a
 @emph{view}.
 
@@ -92,9 +96,30 @@ Provide an initial view for the big-bang.}
 @defproc[(stop-when [stop? ([w world] [dom view] ->  boolean)]) big-bang-handler]{
 Tells @racket[big-bang] the predicate for termination.
 }
-@defproc[(on-tick [tick-f ([w world] [v view] -> world)]) big-bang-handler]{
+@defproc[(on-tick [tick-f ([w world] [v view] [e event]? -> world)]) big-bang-handler]{
 Tells @racket[big-bang] to update the world during clock ticks.
 }
+
+
+@defproc[(on-mock-location-change [location-f ([w world] [v view] [e event]? -> world)]) big-bang-handler]{
+Tells @racket[big-bang] to update the world during simulated movement.
+
+During the extent of a big-bang, a form widget will appear in the
+@tt{document.body} to allow you to manually send location-changing
+events.
+
+}
+
+
+@defproc[(on-location-change [location-f ([w world] [v view] [e event]? -> world)]) big-bang-handler]{
+Tells @racket[big-bang] to update when the location changes, as
+received by the
+@link["http://dev.w3.org/geo/api/spec-source.html"]{Geolocation API}.
+}
+
+
+
+
 @defproc[(to-draw [draw-f ([w world] [v view] -> view)]) big-bang-handler]{
 Tells @racket[big-bang] how to update the rendering of the world.
 }
@@ -133,7 +158,7 @@ Get the textual content at the focus.
 @defproc[(update-view-text [v view] [s string]) view]{
 Update the textual content at the focus.}
 
-@defproc[(view-bind [v view] [type string] [world-updater ([w world] [v view] -> world)]) view]{
+@defproc[(view-bind [v view] [type string] [world-updater ([w world] [v view]  [e event]? -> world)]) view]{
 Attach a world-updating event to the focus.
 
 Common event types include @racket["click"], @racket["trigger"],
@@ -169,6 +194,9 @@ Add the dom node @racket[d] as the last child of the focused node.}
                     
 
 
+@subsection{Events}
+An @deftech{event} is a collection of name-value pairs.
+
 
 
 
diff --git a/web-world/impl.rkt b/web-world/impl.rkt
index ee2634a..27fbe24 100644
--- a/web-world/impl.rkt
+++ b/web-world/impl.rkt
@@ -20,6 +20,12 @@
                     ;; clock tick handler
                     on-tick
 
+                    ;; location changes
+                    on-mock-location-change
+
+                    ;; location changes (for real!)
+                    on-location-change
+                    
                     ;; draw and update the view
                     to-draw
                     
diff --git a/web-world/js-impl.js b/web-world/js-impl.js
index 4cb5796..8fe78be 100644
--- a/web-world/js-impl.js
+++ b/web-world/js-impl.js
@@ -657,6 +657,91 @@
     };
 
 
+
+
+    var MockLocationEventSource = function() {
+        this.elt = undefined;
+    };
+    MockLocationEventSource.prototype = plt.baselib.heir(EventSource.prototype);
+    MockLocationEventSource.prototype.onStart = function(fireEvent) {
+        var mockLocationSetter = document.createElement("div");
+	    
+        var latInput = document.createElement("input");
+        latInput.type = "text";
+	
+        var latOutput = document.createElement("input");
+        latOutput.type = "text";
+        
+        var submitButton = document.createElement("input");
+        submitButton.type = "button";
+        submitButton.value = "send lat/lng";
+        submitButton.onclick = function() {
+            fireEvent(undefined,
+                      { latitude : plt.baselib.numbers.makeFloat(latInput.value),
+                        longitude : plt.baselib.numbers.makeFloat(latOutput.value) });
+            return false;
+        };
+	
+        mockLocationSetter.style.border = "1pt solid black";
+        mockLocationSetter.appendChild(
+            document.createTextNode("mock location setter"));
+        mockLocationSetter.appendChild(latInput);
+        mockLocationSetter.appendChild(latOutput);
+        mockLocationSetter.appendChild(submitButton);
+        document.body.appendChild(mockLocationSetter);
+
+        this.elt = mockLocationSetter;
+    };
+
+    MockLocationEventSource.prototype.onStop = function() {
+        if (this.elt !== undefined) { 
+            document.body.removeChild(mockLocationSetter);
+            this.elt = undefined;
+        };
+    };
+
+
+
+
+
+    // This version really does use the geolocation object.
+    var LocationEventSource = function() {
+        this.id = undefined;
+    };
+
+    LocationEventSource.prototype = plt.baselib.heir(EventSource.prototype);
+
+    LocationEventSource.prototype.onStart = function(fireEvent) {
+        var success = function(position) {
+            fireEvent(undefined,
+                      { latitude : plt.baselib.numbers.makeFloat(position.coords.latitude),
+                        longitude: plt.baselib.numbers.makeFloat(position.coords.longitude) };
+        };
+        var fail = function(err) {
+            // Quiet failure
+        }
+        if (!!navigator.geolocation) {
+            this.id = navigator.geolocation.watchPosition(success, fail); 
+        }
+    };
+    
+    LocationEventSource.prototype.onStop = function() {
+        if (this.id !== undefined) { 
+            navigator.geolocation.clearWatch(this.id);
+            this.id = undefined;
+        };
+    };
+
+
+
+
+
+
+
+
+
+
+
     // DomElementSource: string (U DOM string) -> EventSource
     // A DomEventSource allows DOM elements to send events over to
     // web-world.
@@ -1273,5 +1358,28 @@
 
 
 
+    EXPORTS['on-location-change'] = makePrimitiveProcedure(
+        'on-location-change',
+        1,
+        function(MACHINE) {
+            var onChange = wrapFunction(checkProcedure(MACHINE, 'on-location-change', 0));
+            return new EventHandler('on-location-change', 
+                                    new LocationEventSource(), 
+                                    onChange);
+        });
+
+
+    EXPORTS['on-mock-location-change'] = makePrimitiveProcedure(
+        'on-mock-location-change',
+        1,
+        function(MACHINE) {
+            var onChange = wrapFunction(checkProcedure(MACHINE, 'on-mock-location-change', 0));
+            return new EventHandler('on-mock-location-change', 
+                                    new MockLocationEventSource(), 
+                                    onChange);
+        });
+
+
+
     //////////////////////////////////////////////////////////////////////
 }());
\ No newline at end of file
diff --git a/web-world/racket-impl.rkt b/web-world/racket-impl.rkt
index 64593c2..c1cb35b 100644
--- a/web-world/racket-impl.rkt
+++ b/web-world/racket-impl.rkt
@@ -1,6 +1,10 @@
 #lang racket/base
 
-(provide big-bang initial-view stop-when on-tick to-draw
+(provide big-bang initial-view stop-when
+         on-tick
+         on-location-change on-mock-location-change
+         to-draw
+
          ->view
          view-focus
          view-left view-right view-up view-down
@@ -33,6 +37,22 @@
                [(f delay)
                 (error 'on-tick "Please run in JavaScript context.")]))
 
+
+(define on-location-change
+  (case-lambda [(f)
+                (error 'on-location-change "Please run in JavaScript context.")]
+               [(f delay)
+                (error 'on-location-change "Please run in JavaScript context.")]))
+
+
+(define on-mock-location-change
+  (case-lambda [(f)
+                (error 'on-mock-location-change "Please run in JavaScript context.")]
+               [(f delay)
+                (error 'on-mock-location-change "Please run in JavaScript context.")]))
+
+
+
 (define (to-draw w)
   (error 'to-draw "Please run in JavaScript context."))
 
diff --git a/web-world/where-am-i/index.html b/web-world/where-am-i/index.html
new file mode 100644
index 0000000..d96e4ea
--- /dev/null
+++ b/web-world/where-am-i/index.html
@@ -0,0 +1,9 @@
+<html>
+<head><title>Where in the world am I?</title></head>
+<body>
+<p>
+  I am at: <span id="real-location"/>.
+  The mock location says: <span id="mock-location"/>.
+</p>
+</body>
+</html>
diff --git a/web-world/where-am-i/where-am-i.rkt b/web-world/where-am-i/where-am-i.rkt
new file mode 100644
index 0000000..bd0d3ca
--- /dev/null
+++ b/web-world/where-am-i/where-am-i.rkt
@@ -0,0 +1,28 @@
+#lang planet dyoo/whalesong
+(require (planet dyoo/whalesong/web-world)
+         (planet dyoo/whalesong/resource))
+
+(define-resource index.html)
+
+(define-struct coord (lat lng))
+(define-struct world (real mock))
+
+
+
+(define (location-change world dom evt)
+  world)
+
+
+(define (mock-location-change world dom evt)
+  world)
+
+
+(define (draw world dom)
+  dom)
+
+
+(big-bang (make-world 'unknown 'unknown)
+          (initial-view index.html)
+          (to-draw draw)
+          (on-location-change location-change)
+          (on-mock-location-change mock-location-change))
\ No newline at end of file