Function: alginvol
Section: algebras
C-Name: alginvol
Prototype: DGDG
Help: alginvol({al},{x}): element s(x) where s is the involution stored in al
 (Hamilton quaternions if omitted).
Doc: Given an element $x$ in \var{al} (Hamilton quaternions if omitted) with
 stored involution~$s$, computes $s(x)$ in the algebra \var{al}.
 If~$x$ is omitted, returns the matrix of~$s$.
 For quaternion algebras, the stored involution is the quaternionic conjugation.
 \bprog
 ? al = alginit(nfinit(y), [-1,-1]);
 ? a = algquattobasis(al, [1,2,3,4]~);
 ? b = alginvol(al, a);
 ? algbasistoquat(al, b)
 % = [1, -2, -3, -4]~
 @eprog
