User defined assignment/controlled types

[munnari.oz.au!goanna!pitvax.xx.rmit.edu.au!dale@uunet.uu.net]

In John Barnes report on Ada9x, he states:
---
3.2.2  Assignments and Control Structures
An assignment statement causes the value of a variable to be replaced by that
of an expression of the same type.  Assignment is normally performed by a
simple bit copy of the value provided by the expression.  However, in the case
of non-limited controlled types assignment may be defined by the user.
---

I can't find any information about non limited controlled types in the document
,
nor of how any such user defined assignment would be created. I assume it will
form the basis of the wonderful 'variable' length string package quoated
in the document.

Does anyone know where I can get more info than this?

Thanks in advance
------------------
Dale Stanbrough
RMIT (Royal Melbourne Institute of Technology)
Melbourne Australia.
dale@goanna.cs.rmit.edu.au