function Rx = rot2d(alpha)
% Creates the homogeneous matrix Rx for rotation about the x axis by angle alpha (in degrees)

   Rx = eye(3);
   c  = cosd(alpha);
   s  = sind(alpha);
   
   Rx(1,1) = c;
   Rx(1,2) = s;
   Rx(2,1) = -s;
   Rx(2,2) = c;   
end %function