use strict; 
use Tk;

my $mw = MainWindow->new();
my $input_frame = $mw->Frame()
    ->pack(-expand => 1,
	   -fill => 'both');
my $input = $input_frame->Scrolled('Text')
    ->pack(-expand => 1,
	   -fill => 'both');
$input->tagConfigure("invisible", -elide => 'true');

my %buttons;
for (my $i = 1; $i < 20; $i++) {
    if ($i % 5 == 0) {
	# Create both buttons
	$buttons{"minus_$i"} = $input->Button(-image => $mw->Getimage('minus'),
					      -command => [\&hide,$i]);
	$buttons{"plus_$i"} = $input->Button(-image => $mw->Getimage('plus'),
					      -command => [\&reveal,$i]);
	# Insert only the minus button
	$input->windowCreate('end',-window => $buttons{"minus_$i"});
    }
    $input->insert('end',"abcdefghijkl$i\n");
}

MainLoop(); 


sub hide {
    my ($index) = @_;
    my $from = $index + 1;
    my $to = $index + 5;
    $input->tagAdd("invisible","$from.0","$to.0");
    switch_button($index,'plus','minus');
    return;
}

sub reveal {
    my ($index) = @_;
    my $from = $index + 1;
    my $to = $index + 5;
    $input->tagRemove("invisible","$from.0","$to.0");
    switch_button($index, 'minus', 'plus');
    return;
}

sub switch_button {
    my ($j, $new_sign, $old_sign) = @_;
    $input->delete("$j.0");
    if ($old_sign eq 'plus') {
	$buttons{"plus_$j"} = $input->Button(-image => $mw->Getimage('plus'),
					     -command => [\&reveal,$j]);
    } else {
	$buttons{"minus_$j"} = $input->Button(-image => $mw->Getimage('minus'),
					      -command => [\&hide,$j]);      
    }
    $input->windowCreate("$j.0",-window => $buttons{"${new_sign}_$j"});
    return;
}