The format code
0;
(note the trailing semicolon) should be treated as "two sections, of which the
second is empty", thus negative values should not be displayed when using this
format code. Similar
0;;
should be three sections, negative and zero values not to be displayed.

Currently, a >0;< isn't accepted at all (trailing semicolon gets removed), and
>0;;< is interpreted as two sections, >0;;;< as threee sections.