Function: rnfeltdown
Section: number_fields
C-Name: rnfelementdown
Prototype: GG
Help: rnfeltdown(rnf,x): expresses x on the base field if possible; returns
 an error otherwise.
Doc: $\var{rnf}$ being a relative number
 field extension $L/K$ as output by \kbd{rnfinit} and $x$ being an element of
 $L$ expressed as a polynomial or polmod with polmod coefficients, computes
 $x$ as an element of $K$ as a polmod, assuming $x$ is in $K$ (otherwise an
 error will occur). If $x$ is given on the relative integral basis, apply
 \kbd{rnfbasistoalg} first, otherwise PARI will believe you are dealing with a
 vector.
