A nested module is a module defined in the context of a "father" module and therefore can only be seen in the context of the "father".
After the definition of the Employees and Managers modules as nested modules to the System module, let's define a nested module to the Managers module. We assume that the current module-context is set to the Managers module. Now tell the following frame:
Module TopManagers end
As a result we have defined the nesting hierarchy depicted in Figure 5.2.
Figure 5.2: A simple nesting hierarchy of modules
A nested module can see the content of all its fathers. Therefore when you set the module-context to TopManagers, you can reference all objects contained in the modules TopManagers, Managers and System. When you set the module-context to Managers, you can reference all objects contained in Managers and System.