The Event module provides a central facility to watch for various types
of events and invokes a callback when these events occur.
The idea is to delay the handling of events so that they may be dispatched
in priority order when it is safe for callbacks to execute.

Events (in the ordinary sense of the word) are detected by watchers, which
reify them as events (in the special Event module sense). For clarity, the
former type of events may be called "source events", and the latter
"target events".
Source events, such as signals arriving, happen whether or not they are
being watched. If a source event occurs which a watcher is actively watching
then the watcher generates a corresponding target event.
Target events are only created by watchers.
If several watchers are interested in the same source event then each will
generate their own target event. Hence, any particular source event may
result in zero, one, two, or any number of target events: the same as the
number of watchers which were actively watching for it.

Target events are queued to be processed in priority order (priority being
determined by the creating watcher) and in FIFO order among events of the
same priority.
Queued ("pending") events can, in some cases, be cancelled before being
processed. A queued event is processed by being passed to the callback
function (or method on a particular object or class) which was specified
to the watcher.

A watcher, once created, operates autonomously without the Event user
having to retain any reference to it. However, keeping a reference makes
it possible to modify most of the watcher's characteristics.
A watcher can be switched between active and inactive states.
When inactive, it does not generate target events.

Some types of source event are not reified as target events immediately.
Signals received, for example, are counted initially. The counted signals
are reified at certain execution points. Hence, signal events may be
processed out of order, and if handled carelessly, on the wrong side of
a state change in event handling.
A useful way to view this is that occurrence of the source event is not
actually the arrival of the signal but is triggered by the counting of
the signal.

Reification can be forced when necessary. The schedule on which some
other events are created is non-obvious. This is especially the case
with watchers that watch for a condition rather than an event.
In some cases, target events are generated on a schedule that depends
on the operation of the event loop.