So I filed in the bugs and went to work making sure Tangent can import IEnumerable/IEnumerator. I need them to make foreach work (or perhaps a Python style for). They're also pretty necessary for generators and to do a number of the little example programs I want to get focused on as the driving force behind language implementation rather than my whimsy.
At the moment, the importer gets the types (and ignores their ungeneric kin) and generates the Current property for the enumerator. The others fail out; probably due to generic referencing issues. ~3 more members to get working.