/**
     * process the sample by checking it against each existing invariant and issuing an error if any
     * invariant is falsified or weakened.
     */
    public void process_sample(PptMap all_ppts, PptTopLevel ppt, ValueTuple vt, Integer nonce) {

      this.all_ppts = all_ppts;

      debug.fine("processing sample from: " + ppt.name);

      // Add orig and derived variables
      FileIO.add_orig_variables(ppt, vt.vals, vt.mods, nonce);
      FileIO.add_derived_variables(ppt, vt.vals, vt.mods);

      // Intern the sample
      vt = new ValueTuple(vt.vals, vt.mods);

      // If this is an enter point, just remember it for later
      if (ppt.ppt_name.isEnterPoint()) {
        Assert.assertTrue(nonce != null);
        if (dir_file != null) {
          // Yoav: I had to do a hack to handle the case that several dtrace files are concatenated
          // together,
          // and Sung's dtrace files have unterminated calls, and when concatenating two files you
          // can have the same nonce.
          // So I have to remove the nonce found from the call_map
          call_map.remove(nonce);
        } else Assert.assertTrue(call_map.get(nonce) == null);
        call_map.put(nonce, new EnterCall(ppt, vt));
        debug.fine("Skipping enter sample");
        return;
      }

      // If this is an exit point, process the saved enter point
      if (ppt.ppt_name.isExitPoint()) {
        Assert.assertTrue(nonce != null);
        EnterCall ec = call_map.get(nonce);
        if (ec != null) {
          call_map.remove(nonce);
          debug.fine("Processing enter sample from " + ec.ppt.name);
          add(ec.ppt, ec.vt);
        } else { // didn't find the enter
          if (!quiet)
            System.out.printf("couldn't find enter for nonce %d at ppt %s\n", nonce, ppt.name());
          return;
        }
      }

      add(ppt, vt);
    }