function drawparabola (z, a, alpha)
%DRAWPARABOLA
%
%       drawparabola (z, a, alpha)
%       draw parabola for a*(x - z(1))^2 + (y - z(2)) = 0
%       rotated by alpha
%
  s = sin(alpha); c = cos(alpha);
  Q = [c -s; s c];
  theta = [-10:0.2:10];
  u = diag(z)*ones(2,length(theta)) + ...
      Q*[theta; a*theta.*theta];
  plot(u(1,:),u(2,:));

end % drawparabola

