Ability to check element definition in tag
Imported from: http://groups.google.com/group/in-portal-dev/browse_thread/thread/92fb52a18d4f1963#
In-Portal's template engine's key feature is block definitions. You can create HTML code block using m_DefineElement tag and then use it later using m_RenderElement block. It's like moving widely used PHP code into a function.
However it has no tag for checking if a particular element was defined. Now it is.