A (possibly empty) list of polynomials .
A ``polynomial'' in this case can either be an element of the virtual domain
of polynomials (in the sense of the
description given in Section 4.4,
independent from its structure and representation of ``pure polynomials'',
see Section 4.4.1 and Section 4.4.2) or
a labeled polynomial (see Section 3.14). Functions for labeled
polynomial lists usually carry the tag lpl.
Lists of polynomials are always ordered w.r.t. ascending leading