// This a the rotating hill problem with one turn.// First 1/2 turn is a convection equation and second 1/2 a convection diffusionborder a(t=0, 2*pi)     {    x = cos(t);    y = sin(t);  }; // the unit circlemesh th = buildmesh(a(70));                                 // triangulates the diskfespace Vh(th,P1);func  real hill(real r2){return exp(-10*(r2));}Vh v = hill( (x-0.3)^2 +(y-0.3)^2);                  // initial conditionplot(v);real dt = 0.17,t=0;                                                 // time stepVh u1 = y, u2 = -x;                                        // rotation velocityint i;Vh vv,vo; // work  Finite element function for ( i=0; i< 20 ; i++) {    t += dt;    vo=v;    v=convect([u1,u2],-dt,vo);                          // convect v by u1,u2, dt seconds, results in f // convec(u1,u2,dt,v,v) won't work    plot(v,cmm="convection: t="+t + ", min=" + v[].min + ", max=" +  v[].max,wait=0);};problem  A(v,vv,solver=CG) = int2d(th)(v*vv/dt + 0.01*(dx(v)*dx(vv)+dy(v)*dy(vv)) )  + int2d(th)(-vv*convect([u1,u2],-dt,vo)/dt)  + on(a,v=0);    plot(v,wait=1);