pipelet, emit_operations(), details:
  - implement emit_operations( added, removed, updates [, options ] )

       Emits a number of operations ( add / remove / update ) handling the
       "more" option appropriately to guaranty that:
         1 - Multiple operations of the same batch are grouped using the option
             more
         2 - When there is nothing more to be expected, notify downstream
             pipelets if necessary
  - constructor: initialize this.more to false. This is not strictly necessary
      but allows to mention this important state attribute

  - use emit_operations() for update()