my %tgs;
$tgs{articles}{Python}{Programming}{"Expert's Guide to Scala"}
   = undef; #d3

$tgs{articles}{Perl}
 = "Perl Programming for Biologists"; #d1

$tgs{articles}{Java}
 = "How to Program in Java"; #d1

$tgs{eBooks}{Linux}
 = "Linux: A Beginner's Guide"; #d1