Count the open tasks in the todo example

The left field said how many tasks there were; it now says how many are
still open, "2 of 3", with aup's count(t in tasks where !t.done), which
Pure runs as a fold over the list on every surface. The reference model
counts the same, and in the browser the field follows a task marked done.

1 files changed, 1 insertions, 1 deletions

diff --git a/examples/todo.aup b/examples/todo.aup
index a5ee3b0..f1e3825 100644
--- a/examples/todo.aup
+++ b/examples/todo.aup
@@ -29,7 +29,7 @@ on urgent.down { at.urgent = !at.urgent }
 on drop.down { tasks.remove(at) }
 
 view title = "To do"
-view left = "{tasks.count} tasks"
+view left = "{count(t in tasks where !t.done)} of {tasks.count}"
 
 # A list: label, and optionally tone, badge and detail per row.
 view tasks_list = each task in tasks {