var('x,y')
plot_vector_field((0, 1), (x, -1, 1), (y, -1, 1))
plot_vector_field((x, y), (x, -1, 1), (y, -1, 1))
plot_vector_field((y, x), (x, -1, 1), (y, -1, 1))
plot_vector_field((-y, x), (x, -1, 1), (y, -1, 1))