sub fibonacci {
   my ($n) = @_;
   return 0 if $n == 0;
   return 1 if $n == 1;
   return sum fibonacci($n-2), fibonacci($n-1);
}

##</code><code>##

sub fibonacci {
   my ($n) = @_;
   return 0 if $n == 0;
   return 1 if $n == 1;
   return sum map $_->join(),
      async { fibonacci($n-2) },
      async { fibonacci($n-1) };
}

##</code><code>##

sub fibonacci {
   my ($n) = @_;
   return 0 if $n == 0;
   return 1 if $n == 1;
   return sum map $_->get(),
      async_maybe { fibonacci($n-2) },
      async_maybe { fibonacci($n-1) };
}

##</code><code>##

sub fibonacci {
   my ($n) = @_;
   my $result;
   process_and_wait(sub {
      fibonacci_task(sub { $result = $_[0]; }, $n );
   });
   return $result;
}

sub fibonacci_task {
   my ($on_complete, $n) = @_;
   return $on_complete->(0) if $n == 0;
   return $on_complete->(1) if $n == 1;
   my ($x,$y);
   process(sub { fibonacci_task(sub { $x = $_[0] }, $n-2) });
   process(sub { fibonacci_task(sub { $y = $_[0] }, $n-1) });

   #TODO: The last of the two tasks to complete
   #      must call $on_complete->($x+$y).

}