problem in calling a user-defined package function i an SQL statment

I have created a package function that performs a simple function, similar to Oracle's analytic function row_number() over (partition by <key> order by <key>).
The function uses two persistent variables in a package. If the control variable is the same then a counter is incremented else it resets it to 1. I am currently using version 8.1.7.3 on Solaris

Package foo

   function rn( istring in varchar2) return number; end;
/

Package body foo
MyString varchar2;
MyCounter number;

--
 function  rn( istring in varchar2) return number as
 begin
  if MyString <> istring then
    MyCounter := 1;
    MyString := istring;
  else
   MyCounter := MyCounter + 1;
  end if;
  return MyCounter;
 end;
end;


/


When I call this function in the follwoing SQL statement it work fine:
     select x.*, foo.rn( x.table_type)  RN from (
         select table_name, table_type from cat order by 2,1
     ) x

The result sets are as follows:
Table_name, type_type, RN
TBL_A,    TABLE,    1
TBL_B,    TABLE,    2
VW_A,    VIEW,       1

When I call this function in the following SQL statement it fails:
   select z.* from (
     select x.*, foo.rn( x.table_type)  RN from (
      select table_name, table_type from cat order by 2,1
     ) x
  ) z
 where z.RN <= z.RN;

The result sets are as follows:
Table_name, type_type, RN
TBL_A,    TABLE,    3
TBL_B,    TABLE,    6
VW_A,    VIEW,      3

Conclusion, the reference to variable z.rn actually invokes the function
call instead of evaluating the value from the inner view.
I have tried PRAGMA statements, but that did not help.

Does anybody know why/how this happens?
Thanks for your help, Don