| B | |
| BACKEND [OpamRepository] | |
| G | |
| GenericPackage [OpamTypes] | 
At some point we want to abstract so that the same functions can be used
    over CUDF and OPAM packages
 | 
| I | |
| IO_FILE [OpamFile] | 
All Configuration files satisfies this signature
 | 
| V | |
| VCS [OpamVCS] | 
Each backend should implement this signature.
 |