A marker in { } is optional. If the user does not define optional markers, those markers use ground inertia marker. The user can define a marker name as follows:
(m1,{m2},{m3}) : (bodyname.markername, {bodyname.markername}, {bodyname.markername}).
•m1 becomes action marker.
•If m2 is defined, m2 becomes base marker.
•If m3 is defined, m3 becomes reference marker.