anchor.diff:  
       | task :: tasks, prev :: previous
           when (fst prev) = (update_task_timestamps task) ->
  +      debug "Skipping" task;
         aux env tasks previous (prev :: rev_done)
   
       | tasks, previous ->
  
positive.code:   { tf_fname = fname;
      tf_modtime = t0 }
  
  type dep_task =
    | LDInterleaved of timed_fname * timed_fname (* (interface * implementation) *)
    | LDSingle of timed_fname (* interface or implementation *)
    | LDInterfaceOfCurrentFile of timed_fname (* interface *)
  
  type env_t = DsEnv.env * TcEnv.env
  
  
negative.code:       Inl (env, List.rev rev_done)
  
      | task :: tasks, [] ->
        debug "Loading" task;
        let timestamped_task = update_task_timestamps task in
        (match push_then_complete_task_or_revert env task with
         | Inr env -> Inr (env, List.rev rev_done)
  