The files in this directory require Yves Bertot's "private types"
extension to Coq in order to implement higher inductive types (HITs).
The "private types" extension is included in the stable branch of the
HoTT/coq repository, so if you followed the INSTALL instructions you
should be okay.

The files which use HITs are currently segregated into this directory
because the "private types" extension is a hack whose details may
change in the future, and we are still hoping to one day have an
actual implementation of HITs.
