Module is a simple container of values, types definition and sub modules. There are two types of modules. Local modules and top level ones. The following video clips show how to define them. You can find the source code as part of my F# Fundamentals course. Its community version is available for free personal usage at www.abelski.com.
The Visitor Design Pattern
The visitor design pattern allows us to add operations to objects that already exist without modifying their classes and without extending them.