The type const includes any expr which is of type constant, plus those names which are defined to be const through difforms[defform].

All sums, products, or powers of objects of type const are considered to be const.

The types const, form, and scalar are mutually exclusive.  Any expression should belong to at most one of these types.

This function is part of the difforms package and can only be used after performing the command with(difforms).