Generic type in package

[Stefan Merwitz <thebiggestfan2002@yahoo.de>]

Hello,

I've seen the following type definition in a package-file (ads):

generic
    type T is (<>);

this seems to allow comparing (smaller, greater or equal) of the type T. 
But I did not find this type definition ("(<>") in the Ada 
documentation. What are the differences to limited private and does this 
  constrict the usable data types for T?


Thanks in advance,

Stefan