[Seaside] Ajax callback question: how to update component tree before #children gets called?

[itsme213]

When this anchor is pressed:

html anchor
  onClick: (html updater
    id: someId ;
    callback: [ : r |
      self updateComponentTree.
      self renderSomethingOn: r]);
   with: 'do' ].

I find that #children is called BEFORE my #updateComponentTree.

OK, so I think that #children probably has to be called before 
#renderSomethingOn.

My question: when the anchor is clicked, how do I arrange for
    self updateComponentTree
to execute before #children? Is there a way to have a separate callback for
  - updates to component tree (before #children) Vs.
  - re-rendering callback (after #children)

Thanks,

Sophie