"Must instantiate controlled types at library level." Why?

["Peter C. Chapin" <pchapin@sover.net>]

Hello!

I have a generic package that contains a controlled type. When I attempt to 
instantiate that package in the declarative part of a procedure, GNAT 
(v3.15p) tells me that I must instantiate controlled types at "library 
level" (or something like that). I worked around this by creating a helper 
package in which I did the instantiation and then used that helper package in 
my procedure. However, this experience leads me to wonder why there is this 
restriction on controlled types.

I'm relatively new to Ada so I apologize if this seems like a basic question.

Thanks in advance!

Peter