Problems with 'class, help anyone?

I'm trying to use 'class
here in a way I hope works :)
apparently it doesn't tough..

I'm using example code from:
http://www.ankh-morpork.com/Simon/adabook/x6.htm

Abit old I guess but it's supposed to be for Ada95

I've done almost exactly as this but now I get:
bash-2.03$ gnatmake  main
gcc -c main.adb
main.adb:13:24: no selector "Humanoid" for private type "Instance" defined at mob.ads:7
main.adb:15:27: no selector "Instance" for private type "Instance" defined at mob.ads:7
main.adb:23:18: type of object cannot be abstract
main.adb:28:12: expected type "Pointer" defined at line 8
main.adb:28:12: found private type "Instance" defined at mob-humanoid.ads:11
main.adb:29:12: expected type "Pointer" defined at line 8
main.adb:29:12: found private type "Instance" defined at mob.ads:7
gnatmake: "main.adb" compilation error


According to the webpage above this should work (If I understood it
correctly).

/M�rten Woxberg