[yt-users] creating gradient of a field as a derived field

Hi yt people, I need a little help creating a derived field which will return me the negative gradient of gravitational potential *@derived_field(name = "gradient_gravpot", units = " cm/s**2")* *def _gradient (field, data):* * G = add_gradient_fields(("gas"," gravitational_potental"))* * return (- data["gravitational_potential_gradient_x"])* and I got this error *NameError: global name 'add_gradient_fields' is not defined* please suggest the right method to define the field. thanks Best Turhan