Base class for scope handlers. Scope handlers are dispatch endpoints for expression handling inside a scope, e.g. inside an expression handler routine. As C# doesn't support multiple dynamic dispatch, the expression call the handler themselves which is the foundation of the visitor pattern. As the .NET expression classes don't have a Handle routine, a big switch/case statement is present to overcome this problem. This class contains no real logic, it just dispatches the call to Handle to the right routine which is then overriden in a subclass.