Function: rnfidealdown
Section: number_fields
C-Name: rnfidealdown
Prototype: GG
Help: rnfidealdown(rnf,x): finds the intersection of the ideal x with the
 base field.
Doc: let $\var{rnf}$ be a relative number
 field extension $L/K$ as output by \kbd{rnfinit}, and $x$ an ideal of
 $L$, given either in relative form or by a $\Z$-basis of elements of $L$
 (see \secref{se:rnfidealabstorel}). This function returns the ideal of $K$
 below $x$, i.e.~the intersection of $x$ with $K$.
