function vcross = crmp( v )
% crmp planar cross-product operator (motion).
% crmp(v) calculates the 3x3 matrix such that the expression crmp(v)*m is
% the cross product of the planar motion vectors v and m.
vcross = [ 0 0 0 ;
v(3) 0 -v(1) ;
-v(2) v(1) 0
];