The only way an assistant could get anything onto a diagram was as a side effect of creating an expression - a flag on the call that adds expressions. That means an assistant cannot put a type that already exists onto a diagram, cannot take anything off again, and cannot change what part of the diagram is in view.

Asking the assistant to 'put Customer and its subtypes on the diagram' therefore only works for elements it is creating at that moment, which is rarely what is wanted when reviewing an existing model.

Wanted: adding and removing elements on a diagram, and steering the view, as operations in their own right - working on the diagram the modeller has open, and on the diagram file when nothing is open.