def apply

      #
      # find 'tree'

      tree =
        lookup_val_prefix('tree', :escape => true) ||
        lookup_variable('tree')

      return reply_to_parent(h.applied_workitem) unless tree

      #
      # apply 'tree'

      launch_sub(
        "#{h.fei['expid']}_0", tree,
        :variables => compile_atts(:escape => true))
    end
def apply

      #
      # find 'tree'

      tree =
        lookup_val_prefix('tree', :escape => true) ||
        lookup_variable('tree')

      return reply_to_parent(h.applied_workitem) unless tree

      #
      # apply 'tree'

      launch_sub(
        "#{h.fei['expid']}_0", tree,
        :variables => compile_atts(:escape => true))
    end
TODO : maybe accept directly ruby and xml (and json)
 TODO : _yield ?
 TODO : apply [ 'echo', { 'nada' => nil }, [] ]