Overloading operator “=” for anonymous access types?

[daicrkk@googlemail.com]

I am working my way through Barnes' excellent Ada book. This is a code sample for deep comparison of linked lists from section 11.7:


type Cell is
  record
    Next: access Cell;
    Value: Integer;
  end record;

function "=" (L, R: access Cell) return Boolean is
begin
  if L = null or R = null then    -- universal =
    return L = R;                 -- universal = (Line A)
  elsif L.Value = R.Value then
    return L.Next = R.Next;       -- recurses OK (Line B)
  else
    return False;
  end if;
end "=";


I can't seem to wrap my head around why in Line A operator "=" of the universal_access type is called (because of the preference rule), on Line B, however, the user-defined operator "=" is called (which makes recursion possible in the first place), this time with no preference for operator "=" of universal_access.

Both L and R, as well as L.Next and R.Next are of the same anonymous type "access Cell". Why the difference in "dispatching"? Does it have to do with L and R being access parameters? If so, what is the rule there?

I did my best to find anything in the AARM, especially section 4.5.2, but could not make any sense of it.

Cheers.