Looking at a post by Karl Seguin where he show show designing for testability leads to a better design, in this case, [composition over inheritance](http://openmymind.net/2010/8/19/Design-Through-Testability-An-Example). 

In the example, his `CachedEmployeeLookup` has a dependency on an `EmployeeLookup`, which both implement the same interface, `IEmployeeLookup` 

How you would configure this in StructureMap so that the default class used by the program is `CachedEmployeeLookup` whilst `CachedEmployeeLookup` gets an `EmployeeLookup` injected to it?