function XR = rota(X,a)
% function XR = rota(X,a)
%
% ROTA rotates coordinates by an angle a
%      given in degrees. X should be an n by 2
%      matrix with the x,y pairs.
%      XR contains the rotated x,y pairs.
%
%          Julio Candela VII/93.

ang = a*pi/180 ; cc=cos(ang); ss=sin(ang);
R = [cc -ss;ss cc];
XR = X*R;
