The rules that use the category "<on_prep>" are:

<establish_base_of_fire_command>

<set_up_verb> <base_of_fire_phrase> <location_prep> <engagement_loc> targeting <on_prep> <enemy_loc>

<supporting_fire_command>

support <attack_noun_phrase> <on_prep> <engagement_loc> <by_fire_phrase>

support <by_fire_phrase> <attack_noun_phrase> <on_prep> <engagement_loc>

<open_fire_command_1>

<open_fire_verb> <on_prep> <enemy_loc>