function X = rotx( theta )
% rotx spatial coordinate transform (X-axis rotation).
% rotx(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 X axis.
c = cos(theta);
s = sin(theta);
X = [ 1 0 0 0 0 0 ;
0 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
];