Function curl 

pub fn curl(field: &[Expr], vars: &[&str]) -> Result<[Expr; 3], VecCalcError>

Curl ∇×F of a 3-D field. Requires exactly 3 components and 3 variables [x,y,z]. Returns [ ∂Fz/∂y−∂Fy/∂z , ∂Fx/∂z−∂Fz/∂x , ∂Fy/∂x−∂Fx/∂y ].