Function: nfeltissquare
Section: number_fields
C-Name: nfissquare
Prototype: lGGD&
Help: nfeltissquare(nf,x,{&y}): returns 1 if x is a square in nf (and sets y to a
 square root if present), else returns 0.
Doc: returns $1$ if $x$ is a square in \kbd{nf} (and sets $y$ to a square root if the
 argument is present), else returns 0.

 \bprog
 ? nf = nfinit(1+x^2);
 ? nfeltissquare(nf, -1, &y)
 %2 = 1
 ? y
 %3 = [0, -1]~
 @eprog
