anchor.diff:          dep_graph
           interfaces_needing_inlining
           root_files
  -        for_extraction
           widened
       : list<file_name>
       * bool =
  
positive.code:     all_files,
      widened
  
  let topological_dependences_of
          file_system_map
          dep_graph
  
negative.code:     : list<file_name>
      * bool =
  
      let widened, dep_graph = phase1 file_system_map dep_graph interfaces_needing_inlining for_extraction in
      topological_dependences_of' file_system_map dep_graph interfaces_needing_inlining root_files widened
  
  (** Collect the dependencies for a list of given files.
      And record the entire dependence graph in the memoized state above **)
  