- doubleTypeInference: we check for unreferenced letins in the inferred type also...

- doubleTypeInference: we check for unreferenced letins in the inferred type also after beta-reduction because beta-reduction can cause unreferenced letins
- procedural: bugfix in the use of inner types, the expected type was sometimes used in place of the inferred type; context cleaning is now disabled because the clear tactics are not generated; debugging mode is now activated

nat/ord.ma is now fully reconstructed :)