function E = ry( theta )
% ry 3x3 coordinate rotation (Y-axis)
% ry(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 Y axis.
c = cos(theta);
s = sin(theta);
E = [ c 0 -s;
0 1 0;
s 0 c ];