Extending self directly causes stack overflow on StructuredDataType via allMembers, e.g. ``` Structure st extends st () ``` or ``` Structure st extends stx () Structure stx extends st () ``` Need to guard against extending self (also indirectly). This affects `allMembers`, `allConstraints`, `allGates` , etc. `transitiveExtending` seems to be incorrect as well. * [x] Add constraint * [x] Add guard * [x] Fix and use `transitiveExtending`?