Need some help with Streams

Ok I've found it and it was easier than I suspected:

Function CreateSubStream:TStream(Stream:TStream, Pos:Int, Length:Int, MovePos:Int = False) 
	Local OldPos:Int = stream.Pos() 
	?debug
		If Stream = Null Then
			Throw "Stream was null."
		End If
		If pos < 0 Or pos > stream.Size() Then
			Throw "Byte position out of bounds."
		End If
	?
	Try
		stream.Seek(pos) 
	Catch Err:String
		Throw "Stream has to be seekable."
	End Try
	Local B:TBank = CreateBank(Length) 
	ReadBank(b, stream, 0, length) 
	Local NewStream:TStream = OpenStream(B) 
	If MovePos = False Then Stream.Seek(oldpos) 
	Return newstream
End Function

the best thing is that there's no need to create a new class, as I suspected initially, so this way I think it would be much more easy to mantain if there's any bug there.