'double' nest form fields with accepts_nested_attributes_for

Hi

Is it possible to 'double' nest form fields with
accepts_nested_attributes_for?
Something like:
[code=ruby]<% form_for parent do |parent_form| %>
  <% parent_form.fields_for parent_children do |parent_children_fields|
%>
      ...
         <% parent_children_fields.fields_for parent_child_activities do

parent_child_activities_fields| %>

             ...[/code]
Rough example but should be enough for demonstration. What I'm trying to
do is form a many to many join between parent and child while
simultaneously creating a join to their activities. A parent has many
children. A child has many parents. Parents and children together have
many activities.

If this would be possible it would be a wonderful thing, though it
depends on creation of the first level join before the second. Anyone
have experience of this?