function animation( tf, nt)
    
    nx, ny = 64, 64
    xmin, xmax, nx = -π, π, nx
    ymin, ymax, ny = -π, π, ny
    mesh = Mesh(xmin, xmax, nx, ymin, ymax, ny)
    f  = zeros(Float64,(nx,ny))
    dt = tf / nt
    bar = Progress(nt,1) ## progress bar
    t = 0
    @gif for n=1:nt
       
       f .= exact(t, mesh)
       t += dt
       p = contour(mesh.x, mesh.y, f)
       plot!(p[1]; clims=(0.,1.), aspect_ratio=:equal)
       plot!(sqrt(2) .* cos.(-pi:0.1:pi+0.1), sqrt(2) .* sin.(-pi:0.1:pi+0.1))
       next!(bar) ## increment the progress bar
        
    end
    
end

animation( 2π, 100)