code: function reserve_and_commit_one (shared_ctx, self_ctx, cb) {
    self_ctx.pop_count ++;
  
    shared_ctx.q.pop (self_ctx.consumer, shared_ctx.pop_opts, (err, res) => {
      if (err) return cb (err);
      self_ctx.resv_count ++;
  
      das_pop++;
  
      if (chance.bool({likelihood: 40})) {
        shared_ctx.q.ok (res._id, (err) => {
          if (err) return cb (err);
          self_ctx.ok_count ++;
          shared_ctx.pop_count ++;
          das_ok++;
  
          if (shared_ctx.pop_count > shared_ctx.pop_max) {
            console.log ('rcr consumer %s ended', self_ctx.id);
            shared_ctx.q.cancel ();
            return cb ();
          }
  
  //        if ((shared_ctx.pop_count % 100000) == 0) console.log ('%s : consumed #%d elems', self_ctx.id, shared_ctx.pop_count);
          reserve_and_commit_one (shared_ctx, self_ctx, cb);
        });
      }
      else {
        shared_ctx.q.ko (res._id, (err) => {
          if (err) return cb (err);
          self_ctx.ko_count ++;
          das_ko++;
  
  //        if ((shared_ctx.pop_count % 100000) == 0) console.log ('%s : consumed #%d elems', self_ctx.id, shared_ctx.pop_count);
          reserve_and_commit_one (shared_ctx, self_ctx, cb);
        });
      }
    });
  }
docs: ////////////////////////////////////////////////////////////////////////////////////////////////////////////