Here is an example of abstract sets implemented by both arrays and lists,
Gnat (3.01 - win95) seems to have problems dispatching the 'unit' call in the convert subprogram.
Another problem is that it needs to controlled types to be declared at the library level -
any ideas ?

I enclose the sources of 9 units:
abstract_sets - spec and body
array_sets - spec and body
list_sets - spec and body
convert - convert routine - here is the 'unit' error
try - a simple try program - problem with controlled types.