function [rotate] = rot_z (ang)
%     ---------------------------------------
%     orthogonal rotation matrix about z-axis
%     ---------------------------------------
%     ang    = angle of rotation about z-axis in radians
%     rotate = rotation matrix (sub-set of finite)
      rotate = eye(3)   ;
      c      = cos(ang);
      s      = sin(ang);
      rotate(1,1) = c;
      rotate(2,1) = s;
      rotate(1,2) = -s;
      rotate(2,2) = c;
%     end of rot_z