[Spark] Converting Arrays

[Lutz Donnerhacke <lutz@iks-jena.de>]

I run into a difficult problem (for me):
$ cat test.ada <<END
package test is
   type Fullpath is array(Positive range <>) of Character; 
   procedure To_Fullpath (s : String; path : out Fullpath);    
   --# derives path from s;       
end test;

package body test is
   procedure To_Fullpath (s : String; path : out Fullpath) is 
   begin    
      path := Fullpath'(others => ASCII.NUL);       
      for i in Positive range 1 .. s'Length loop             
        path (path'First + i) := s (s'First + i);        
      end loop;
   end To_Fullpath;
end test;
END
$ spark test.ada
          *******************************************************
       SPARK95 Examiner with VC and RTC Generator Release 6.3 / 11.02
                           Demonstration Version
          *******************************************************


                       DATE : 10-MAR-2003 18:05:17.49


           Examining the specification of package test ...

           Examining the body of package test ...

  10        path := Fullpath'(others => ASCII.NUL);
            ^
***   Semantic Error    : 39: Illegal use of unconstrained type.
                    ^
***   Semantic Error    : 39: Illegal use of unconstrained type.
                    ^
***   Semantic Error    : 39: Illegal use of unconstrained type [Barnes 6.7].

           Generating listing file test.lst ...

           Generating report file ...


-----------End of SPARK Examination--------------------------------


Any idea, how to write this correctly?