# col2label (4) => "D"
sub col2label {
    ref $_[0] eq __PACKAGE__ and shift;
    my $c = shift;
    defined $c && $c > 0 or return "";
    my $cell = "";
    while ($c) {
        use integer;

        substr $cell, 0, 0, chr (--$c % 26 + ord "A");
        $c /= 26;
        }
    $cell;
    } # col2label