[How to][Trema] Get Flow_Removed message in routing_switch app

I asked an question about how to get Flow_Removed message in routing_switch app. But, soon I realized that I asked a stupid one because the answer is also in OpenFlow Spec v1.0.0.
We should set a "OFPFF_SEND_FLOW_REM" flag into "Flow Mod" message when we want to set a flow. The following list is the discussion message log.

================================================================ 
Hi,


If you want to receive "Flow Removed" message, you have to set
"OFPFF_SEND_FLOW_REM" flag into "Flow Mod" message when you set the flow.


Currently "routing_switch" app does not set this flag, then switches does
not send "Flow Removed" message.

================================================================
Hi,
  I try to get "Flow_Removed" message in routing_switch app, but do not see any one coming in.
The following is what I have done in the app and could someone tell me how to fix or modify it?
Many thanks.


Add it in main():
  set_flow_removed_handler( handle_flow_removed, routing_switch );


Add a method for display:
void handle_flow_removed( uint64_t datapath_id, flow_removed message ) {
  routing_switch *routing_switch = message.user_data;
  info( "[handle_flow_removed] dpid = %#" PRIx64 ", match = %s",
          datapath_id, message.match );
}


Modify routing_switch.conf:
vswitch {
  datapath_id "0xe0"
}
vhost ("host1") {
  ip "192.168.0.1"
  netmask "255.255.0.0"
  mac "00:00:00:01:00:01"
}
vhost ("host2") {
  ip "192.168.0.2"
  netmask "255.255.0.0"
  mac "00:00:00:01:00:02"
}
link "0xe0", "host1"
link "0xe0", "host2"


run {
  path "../apps/topology/topology"
}
run {
  path "../apps/topology/topology_

discovery"
}
run {
  path "../apps/routing_switch/routing_switch"
}


event :port_status => "topology", :packet_in => "filter", :state_notify => "topology", :flow_removed => "routing_switch"
filter :lldp => "topology_discovery", :packet_in => "routing_switch"