code: function registerHandler(type, handler) {
    if (handler[type]) {
      throw new Error('Attempted to define handler for ' + type + ' twice');
    }
  
    handlers[type] = handler;
  }
docs: Handlers consume and store a stream producing a receipt which can then be used to manipulate the stored instance of the data. Each handler must expose the follow methods:  - store - Consumes stream, produces a promise which resolves to the receipt - retrieve - Consumes a receipt produced by store and returns a stream corresponding to the original data - destroy - Consumes a receipt and deletes the data.