Curl of a vector field.
与 .wl 参考的差异： Higher-rank `Curl`
Curl[{f1, f2}, {x1, x2}]
Curl[{f1, f2, f3}, {x1, x2, x3}]
Curl[f, {x1, …, xn}]
Curl[f, x, chart]
Curl[{y, -x, 0}, {x, y, z}]
→ {0, 0, -2}Curl[{0, r}, {r, t}, "Polar"]
→ 2Curl[{0, 0, r}, {r, t, z}, "Cylindrical"]
→ {0, -1, 0}