function E = rx( theta )
% rx 3x3 coordinate rotation (X-axis)
% rx(theta) calculates the 3x3 rotational coordinate transform matrix from
% A to B coordinates, 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);
E = [ 1 0 0;
0 c s;
0 -s c ];