function r=rot4x(theta)
% r = rotx(theta)
%
% rotx produces a 4x4 rotation matrix representing
% a rotation by theta radians about the x axis.
%
%	Argument definitions:
%	
%	theta = rotation angle in radians
c = cos(theta);
s = sin(theta);
r = [1  0  0 0;
     0  c -s 0;
     0  s  c 0;
     0  0  0 1];
