A user should be able to cast an element (or an attribute) to another type. It should be done in a way similar to the
way predicates are added now -- right-click on a node and choose "Cast to...". He is then presented with a type chooser,
where he selects a derived type (or a restricted one in case of simple types) and an additional node is created next to
the original.

The type cat information is persisted in an extension element on the process level so that the cast information is
directly available in all mappers across the process. When the cast is actually used in a mapping a reference to the
cast is added to an extension in the copy element.