generalize (tunrec τ); intros. case_eqH; intros.
- destructp; simplin *. destruct (sublist_decsl (domainl)).
+ exact (RecMaybeClosed (rprojectlsl)). (* This is always a closed record *)
+ exactNone. (* It is only well typed when sl is a sublist of domain l *)
- exactNone.
Defined.