function X = Xrotz( theta )
% Xrotz spatial coordinate transform (Z-axis rotation).
% Xrotz(theta) calculates the coordinate transform matrix from A to B
% coordinates for spatial motion vectors, where coordinate frame B is
% rotated by an angle theta (radians) relative to frame A about their
% common Z axis.
c = cos(theta);
s = sin(theta);
X = [ c s 0 0 0 0 ;
-s c 0 0 0 0 ;
0 0 1 0 0 0 ;
0 0 0 c s 0 ;
0 0 0 -s c 0 ;
0 0 0 0 0 1
];