There's something about polymorphism in Ada that I don't understand. (Or maybe it's the package naming?) I'm trying to parse an XML file using xmlada library and I followed the example I found in tests for this library that uses Debug_Reader. Below is the problematic code:

--  cog_cli-xml.adb
with Ada.Strings.Unbounded;
with Sax.Readers;
with Input_Sources.Strings;
with Unicode.CES.Basic_8bit;
with Cog_Cli.Xml_Reader;
with Cog_Cli_Doc;

package body Cog_Cli.Xml is
   
   package Asu renames Ada.Strings.Unbounded;
   package Ccxr renames Cog_Cli.Xml_Reader;
   package Sr renames Sax.Readers;
   package Iss renames Input_Sources.Strings;
   package Ccd renames Cog_Cli_Doc;
   package U8bit renames Unicode.CES.Basic_8bit;

   function Cli_Help (Command : String) return String is
      Doc_Reader : Ccxr.Reader;
      Input      : Iss.String_Input;
      Cli_Help   : constant Ccd.Content_Type := Ccd.Get_Content ("cli.xml");
   begin
      Iss.Open (Cli_Help.Content.all,
                Encoding => U8bit.Basic_8bit_Encoding,
                Input => Input);
      Ccxr.Set_Command (Doc_Reader, Command);

      Sr.Set_Feature (Doc_Reader, Sr.Namespace_Prefixes_Feature, False);
      Sr.Set_Feature (Doc_Reader, Sr.Namespace_Feature, False);
      Sr.Set_Feature (Doc_Reader, Sr.Validation_Feature, False);

      Sr.Parse (Doc_Reader, Input);

      Iss.Close (Input);

      return Asu.To_String (Doc_Reader.Help);
   end Cli_Help;

end Cog_Cli.Xml;

--  cog_cli-xml_reader.ads
with Sax.Readers;
with Unicode.CES;
with Sax.Attributes;
with Ada.Strings.Unbounded;

package Cog_Cli.Xml_Reader is
   
   package Asu renames Ada.Strings.Unbounded;

   type Reader is new Sax.Readers.Reader with private;

   procedure Start_Element
     (Handler       : in out Reader;
      Namespace_URI : Unicode.CES.Byte_Sequence := "";
      Local_Name    : Unicode.CES.Byte_Sequence := "";
      Qname         : Unicode.CES.Byte_Sequence := "";
      Atts          : Sax.Attributes.Attributes'Class);

   procedure End_Element
     (Handler       : in out Reader;
      Namespace_URI : Unicode.CES.Byte_Sequence := "";
      Local_Name    : Unicode.CES.Byte_Sequence := "";
      Qname         : Unicode.CES.Byte_Sequence := "");

   procedure Characters
     (Handler : in out Reader;
      Ch      : Unicode.CES.Byte_Sequence);

   procedure Set_Command (Handler : in out Reader; Command : String);

   procedure Set_Help (Handler : in out Reader; Help : String);
   
private
   type Reader is new Sax.Readers.Reader with record
      Command : Asu.Unbounded_String;
      Help    : Asu.Unbounded_String;
   end record;

end Cog_Cli.Xml_Reader;

with Ada.Text_IO;
with Ada.Strings.Unbounded;

package body Cog_Cli.Xml_Reader is
   
   package Ati renames Ada.Text_IO;
   package Asu renames with Ada.Strings.Unbounded;

   procedure Start_Element
     (Handler       : in out Reader;
      Namespace_URI : Unicode.CES.Byte_Sequence := "";
      Local_Name    : Unicode.CES.Byte_Sequence := "";
      Qname         : Unicode.CES.Byte_Sequence := "";
      Atts          : Sax.Attributes.Attributes'Class) is
   begin
      null;
   end Start_Element;
   
   procedure Characters
     (Handler : in out Reader;
      Ch      : Unicode.CES.Byte_Sequence) is
   begin
      null;
   end Characters;
   
   procedure End_Element
     (Handler       : in out Reader;
      Namespace_URI : Unicode.CES.Byte_Sequence := "";
      Local_Name    : Unicode.CES.Byte_Sequence := "";
      Qname         : Unicode.CES.Byte_Sequence := "") is
   begin
      null;
   end End_Element;

   procedure Set_Command (Handler : in out Reader; Command : String) is
   begin
      Handler.Command := Asu.To_Unbounded_String (Command);
   end Set_Command;

   procedure Set_Help (Handler : in out Reader; Help : String) is
   begin
      Handler.Help := Asu.To_Unbounded_String (Help);
   end Set_Help;

end Cog_Cli.Xml_Reader;

The problem is that Sr.Set_Feature doesn't believe that Doc_Reader is the right type... I have no idea why. The literal error I'm getting is:

cog_cli-xml.adb:27:09: error: no candidate interpretations match the actuals:
cog_cli-xml.adb:27:23: error: expected private type "Readers.Reader" defined at sax-readers.ads:778
cog_cli-xml.adb:27:23: error: found private type "Xml_Reader.Reader" defined at cog_cli-xml_reader.ads:10