my $ent1 = $table->Entry (
                -font     => "verdana 10",
                -validate => 'key',
                # Pass button widget and a ref to $filled into callback. Need a
                # ref because we manipulate $filled in the callback
                -vcmd     => [\&onEdit, $press, \$filled],
            )

##</code><code>##

sub onEdit {
    my ($button, $filled, $new, $edit, $current, $idx, $type) = @_;
    return 1 if $new eq $current;

##</code><code>##

    if (length $new && !length $current) {
        ++$$filled;
    } elsif (!length $new && length $current) {
        --$$filled;
    }

##</code><code>##

    if ($$filled == kRows * kCols) {
        $button->configure (-state => 'normal');
    } else {
        $button->configure (-state => 'disabled');
    }