The category "<supporting_fire_command>" can be expressed as follows:

<supporting_fire_command>

<support_verb> <np-gapsout_null-gapsin_null-attack>

<support_verb-not_fire> <by_fire_phrase> <np-gapsout_null-gapsin_null-attack>

<support_verb-not_fire> <np-gapsout_null-gapsin_null-attack> <by_fire_phrase>