Function: nfeltpow
Section: number_fields
C-Name: nfpow
Prototype: GGG
Help: nfeltpow(nf,x,k): element x^k in nf.
Doc: given an element $x$ in \var{nf}, and a positive or negative integer $k$,
 computes $x^{k}$ in the number field $\var{nf}$.
Variant: \fun{GEN}{nfinv}{GEN nf, GEN x} correspond to $k = -1$, and
 \fun{GEN}{nfsqr}{GEN nf,GEN x} to $k = 2$.
