The rules that use the vocabulary item "center" are:
center <compass_direction-gapsout_null-gapsin_null> of <zoomable_thing-gapsout_null-gapsin_null>
center <zoomable_thing-gapsout_null-gapsin_null>
center in on <zoomable_thing-gapsout_null-gapsin_null>
center
center <compass_direction-gapsout_null-gapsin_null> of
center in on
center on
center terrain at standard scale
center terrain standard scale