p_Invers: is only a helper routine for the 3-argument forms of jet
(see https://www.singular.uni-kl.de/Manual/4 ... ng_265.htm
It should not be used otherwise: is a static routine in newer releases
Furthermore, the coefficients must be from a field.
p_IsUnit: is currently only used to simplify ideals,
and currently defined as: p is a constant polynomial and the constant is a unit.
I will try to extend that.....but it is of low priority.