Function: nfeltpowmodpr
Section: number_fields
C-Name: nfpowmodpr
Prototype: GGGG
Help: nfeltpowmodpr(nf,x,k,pr): element x^k modulo pr in nf, where pr is in
 modpr format (see nfmodprinit).
Doc: given an element $x$ in \var{nf}, an integer $k$ and a prime ideal
 \var{pr} in \kbd{modpr} format
 (see \tet{nfmodprinit}), computes $x^k$ modulo the prime ideal \var{pr}.
Variant: This function is normally useless in library mode. Project your
 inputs to the residue field using \kbd{nf\_to\_Fq}, then work there.
