Function: sign (<expr>) Attempts to determine the sign of <expr>
on the basis of the facts in the current data base. It returns
one of the following answers: pz (positive or zero), nz
(negative or zero), pn (positive or negative), or pnz (positive,
negative, or zero, i.e. nothing known). There are also some
inexact matches for ?? sign to see them. (%o1) true (%i2)