Function: algeltfromnf
Section: algebras
C-Name: algeltfromnf
Prototype: GG
Help: algeltfromnf(al,x): returns x as an element of al, where x belongs to the
 center of al, a number field.

Doc: Given a central simple algebra \var{al} output by \tet{alginit}, and an
 element \var{x} of its center, returns \var{x} as an element of \var{al}.
 \bprog
 ? A = alginit(nfinit(y^2-5),[2,y]);
 ? z = algeltfromnf(A, 1+2*y)
 % = [3, 4, 0, 0, 0, 0, 0, 0]~
 ? algtrace(A,z)
 % = Mod(4*y + 2, y^2 - 5)
 @eprog
