The rules that use the vocabulary item "is" are:

<confirmation_relation-sing>

is

<zoom_operation-gapsout_null-gapsin_null>

where is <zoomable_thing-gapsout_null-gapsin_null>

<cop-sing>

is

<controller-air_traffic_control>

this is <controller-air_traffic_control-recursive>

<contact_rel>

is

is at

<controller>

this is <controller-air_traffic_control-recursive>

this is <controller-recursive>