def apply
      update_tree
      h.updated_tree[1]['global'] = true
      h.updated_tree[1]['left_tag'] = tag_from_attribute
      h.updated_tree[1]['merge'] = 'drop'
      super
    end
def apply
      update_tree
      h.updated_tree[1]['global'] = true
      h.updated_tree[1]['left_tag'] = tag_from_attribute
      h.updated_tree[1]['merge'] = 'drop'
      super
    end
This does the same as the base AwaitExpression#apply, except that this
 will always be a global listener, listening for a 'left_tag' event, and
 the event's workitem will be discarded after the reply is complete.  The
 event's workitem is only used for comparisons in the where clause (see
 #reply).