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      : $_[0]{_ae_sent}[0]
}

sub cb {
   my $cv = shift;

   @_
      and $cv->{_ae_cb} = shift
      and $cv->{_ae_sent}
      and (delete $cv->{_ae_cb})->($cv);

   $cv->{_ae_cb}
}

sub begin {
   ++$_[0]{_ae_counter};
   $_[0]{_ae_end_cb} = $_[1] if @_ > 1;
}

sub end {
   return if --$_[0]{_ae_counter};
   &{ $_[0]{_ae_end_cb} || sub { $_[0]->send } };
}

# undocumented/compatibility with pre-3.4
*broadcast = \&send;
*wait      = \&recv;

=head1 ERROR AND EXCEPTION HANDLING

In general, AnyEvent does not do any error handling - it relies on the
caller to do that if required. The L<AnyEvent::Strict> module (see also
the C<PERL_ANYEVENT_STRICT> environment variable, below) provides strict
checking of all AnyEvent methods, however, which is highly useful during
development.

As for exception handling (i.e. runtime errors and exceptions thrown while
executing a callback), this is not only highly event-loop specific, but
also not in any way wrapped by this module, as this is the job of the main
program.

The pure perl event loop simply re-throws the exception (usually
within C<< condvar->recv >>), the L<Event> and L<EV> modules call C<<
$Event/EV::DIED->() >>, L<Glib> uses C<< install_exception_handler >> and
so on.

=head1 ENVIRONMENT VARIABLES

AnyEvent supports a number of environment variables that tune the
runtime behaviour. They are usually evaluated when AnyEvent is
loaded, initialised, or a submodule that uses them is loaded. Many of
them also cause AnyEvent to load additional modules - for example,
C<PERL_ANYEVENT_DEBUG_WRAP> causes the L<AnyEvent::Debug> module to be
loaded.

All the environment variables documented here start with
C<PERL_ANYEVENT_>, which is what AnyEvent considers its own
namespace. Other modules are encouraged (but by no means required) to use
C<PERL_ANYEVENT_SUBMODULE> if they have registered the AnyEvent::Submodule
namespace on CPAN, for any submodule. For example, L<AnyEvent::HTTP> could
be expected to use C<PERL_ANYEVENT_HTTP_PROXY> (it should not access env
variables starting with C<AE_>, see below).

All variables can also be set via the C<AE_> prefix, that is, instead
of setting C<PERL_ANYEVENT_VERBOSE> you can also set C<AE_VERBOSE>. In
case there is a clash btween anyevent and another program that uses
C<AE_something> you can set the corresponding C<PERL_ANYEVENT_something>
variable to the empty string, as those variables take precedence.

When AnyEvent is first loaded, it copies all C<AE_xxx> env variables
to their C<PERL_ANYEVENT_xxx> counterpart unless that variable already
exists. If taint mode is on, then AnyEvent will remove I<all> environment
variables starting with C<PERL_ANYEVENT_> from C<%ENV> (or replace them
with C<undef> or the empty string, if the corresaponding C<AE_> variable
is set).

The exact algorithm is currently:

   1. if taint mode enabled, delete all PERL_ANYEVENT_xyz variables from %ENV
   2. copy over AE_xyz to PERL_ANYEVENT_xyz unless the latter alraedy exists
   3. if taint mode enabled, set all PERL_ANYEVENT_xyz variables to undef.

This ensures that child processes will not see the C<AE_> variables.

The following environment variables are currently known to AnyEvent:

=over 4

=item C<PERL_ANYEVENT_VERBOSE>

By default, AnyEvent will log messages with loglevel C<4> (C<error>) or
higher (see L<AnyEvent::Log>). You can set this environment variable to a
numerical loglevel to make AnyEvent more (or less) talkative.

If you want to do more than just set the global logging level
you should have a look at C<PERL_ANYEVENT_LOG>, which allows much more
complex specifications.

When set to C<0> (C<off>), then no messages whatsoever will be logged with
everything else at defaults.

When set to C<5> or higher (C<warn>), AnyEvent warns about unexpected
conditions, such as not being able to load the event model specified by
C<PERL_ANYEVENT_MODEL>, or a guard callback throwing an exception - this
is the minimum recommended level for use during development.

When set to C<7> or higher (info), AnyEvent reports which event model it
chooses.

When set to C<8> or higher (debug), then AnyEvent will report extra
information on which optional modules it loads and how it implements
certain features.

=item C<PERL_ANYEVENT_LOG>

Accepts rather complex logging specifications. For example, you could log
all C<debug> messages of some module to stderr, warnings and above to
stderr, and errors and above to syslog, with:

   PERL_ANYEVENT_LOG=Some::Module=debug,+log:filter=warn,+%syslog:%syslog=error,syslog