anchor.diff:        if cmd_line_has_impl key //unless the cmd line contains 'a.fst'
         && Option.isNone (Options.dep()) //and we're not just doing a dependency scan using `--dep _`
         then if Options.expose_interfaces()
  -           then maybe_add_suffix (Option.get (implementation_of file_system_map key))
  +           then maybe_use_cache_of (Option.get (implementation_of file_system_map key))
              else raise_err (Errors.Fatal_MissingExposeInterfacesOption,
                              BU.format3 "You may have a cyclic dependence on module %s: use --dep full to confirm. \
                                          Alternatively, invoking fstar with %s on the command line breaks \
  
positive.code:            is_implementation fn &&
             key = lowercase_module_name fn)
      in
      let maybe_add_suffix f = if use_checked_file then cache_file_name f else f in
      match d with
      | UseInterface key ->
        //This key always resolves to an interface source file
  
negative.code:                                        key
                                         (Option.get (implementation_of file_system_map key))
                                         (Option.get (interface_of file_system_map key)))
        else maybe_use_cache_of (Option.get (interface_of file_system_map key))   //we prefer to use 'a.fsti'
  
      | PreferInterface key
      | UseImplementation key
  