# Monitor the arguments of subroutines ...
my $monitor = monitor_arg
	# ... in the following packages ...
    in_package => [ 'Foo', 'Bar', 'Ni' ],
	# ... where the argument isa Foo object
    matching => sub { ref($_[0]) && UNIVERSAL::isa($_[0], 'Foo') },
    # ... and check that we didn't mess with the internals ...
    with => sub {
        my ($original, $current, $subroutine) = @_;
        my $removed = $original->todo - $current->todo;
        my $done = $current->done - $original->done;
        warn "removed item not added to done list in $subroutine\n"
            if $removed > 0 && $removed != $done;
    };