Sequence is the only public API today to SET substitution element instance and GET the substitution element instance.
I am proposing mapping substituted property to Sequence, e.g. "implementation" property returns Sequence INSTEAD OF a normal type. If we can do that, SDO users are enabled to set and get substitution element instance which can't be done today. After we do that, we can hide/discard "implementationGroup" or just delegate "implementation" to "implementationGroup". I guess the proposal needs to change the spec, unfortunately. -- Sincerely Yang ZHONG
