Rider = POINTER TO ABSTRACT RECORD
  (r: Rider) Available (): LONGINT, NEW, ABSTRACT
END;

Scroller = POINTER TO ABSTRACT RECORD
  (s: Scroller) MinPosition (): LONGINT, NEW, ABSTRACT;
  (s: Scroller) MaxPosition (): LONGINT, NEW, ABSTRACT;
  (s: Scroller) GetPosition (r: Rider): LONGINT, NEW, ABSTRACT;
  (s: Scroller) SetPosition (r: Rider; position: LONGINT), NEW, EMPTY
END;

Reader = POINTER TO ABSTRACT RECORD (Rider)
  (r: Reader) ReadByte (OUT a: BYTE), NEW, ABSTRACT;
  (r: Reader) ReadBytes (OUT a: ARRAY OF BYTE; offset, count: INTEGER), NEW, ABSTRACT
END;

Writer = POINTER TO ABSTRACT RECORD (Rider)
  (w: Writer) WriteByte (a: BYTE), NEW, ABSTRACT;
  (w: Writer) WriteBytes (IN a: ARRAY OF BYTE; offset, count: INTEGER), NEW, ABSTRACT
END;

Buffer = POINTER TO ABSTRACT RECORD
  (b: Buffer) NewReader (old: Reader): Reader, NEW, ABSTRACT;
  (b: Buffer) NewWriter (old: Writer): Writer, NEW, ABSTRACT;
  (b: Buffer) Scroller (): Scroller, NEW, ABSTRACT
END;

Channel = POINTER TO ABSTRACT RECORD
  (c: Channel) Reader (): Reader, NEW, ABSTRACT;
  (c: Channel) Writer (): Writer, NEW, ABSTRACT;
END;

Scroller = POINTER TO ABSTRACT RECORD
  (s: Scroller) MinPosition (r: Rider): LONGINT, NEW, ABSTRACT;
  (s: Scroller) MaxPosition (r: Rider): LONGINT, NEW, ABSTRACT;
  (s: Scroller) GetPosition (r: Rider): LONGINT, NEW, ABSTRACT;
  (s: Scroller) SetPosition (r: Rider; position: LONGINT), NEW, EMPTY
END;

public abstract class Rider
{
   public virtual object Locator { get { return null; } }
}

public abstract class Writer: Rider
{
   public abstract void WriteByte (byte b);
   public abstract void WriteBytes (byte[] bytes, int offset, int count);
   public virtual void WriteBoolean (bool x)
   ...
}

public abstract class Reader: Rider
{
   public abstract long Available { get; }
   public abstract byte ReadByte ();
   public abstract void ReadBytes (byte[] bytes, int offset, int count);
   public virtual bool ReadBool ()
   ...
}

public abstract class Scroller
{
   public virtual long MinPosition (Rider r) { return 0; }
   public virtual long MaxPosition (Rider r) { return System.Int64.MaxValue; }
   public abstract long GetPosition (Rider r);
   public abstract void SetPosition (Rider r, long position);
}

public abstract class Buffer
{
   public abstract long Length { get; }
   public abstract Scroller Scroller { get; }
   public abstract Writer NewWriter ();
   public abstract Reader NewReader ();
}

private sealed class TheScroller: Scroller
{
   private static Locator GetLocator (Rider r)
   {
      Locator loc = r.Locator as Locator;
      if (loc != null) return loc; else throw new System.Exception("Alien rider.");
   }
   public override long MaxPosition (Rider r)
   {
      return GetLocator(r).buffer.length;
   }
   public override long GetPosition (Rider r)
   {
      return GetLocator(r).Position;
   }
   public override void SetPosition (Rider r, long position)
   {
      GetLocator(r).Position = position;
   }
}