function [u,v] = swirlfunc(x,y)
% rotfunc: rotate (x,y) by a radius-dependent angle.
% Rotate by theta = r e^-r.
r = sqrt(x.^2 + y.^2);
rer = 2*r .* exp(-r);
c = cos(rer);
s = sin(rer);
u = c.*x - s.*y;
v = s.*x + c.*y;
