The function d computes the exterior derivative of an expression. If the expression is a list, then d is applied to each element in the list.

When d is called with an expression and a list of 1-forms, any name of type scalar in the expression will be expanded in these 1-forms.  It is assumed that the 1-forms are independent.  For each 1-form, a new scalar is created to be the component for that 1-form.

The command with(difforms,d) allows the use of the abbreviated form of this command.