ANCHOR_RELATION_LABEL moves the anchor point of an existing
relation label.
--->(ANCHOR_RELATION_LABEL)--|pick label|--specify position|--->
This action has no options.
The following parameter sequence moves the anchor point
of an existing relation label to a new position.
ANCHOR_RELATION_LABEL [pick label] [specify changed anchor point position]
CREATE_RELATION terminate action
SOLVE_RELATION_SET terminate action
SET_RELATIONS terminate action
ANIMATE_RELATION terminate action
SET_CURRENT_REL_SET terminate action