Function: algneg
Section: algebras
C-Name: algneg
Prototype: GG
Help: algneg(al,x): element -x in al.
Doc: Given an element $x$ in \var{al}, computes its opposite $-x$ in the
 algebra \var{al}.
 \bprog
 ? A = alginit(nfinit(y), [-1,-1]);
 ? algneg(A,[1,1,0,0]~)
 %2 = [-1, -1, 0, 0]~
 @eprog

 Also accepts matrices with coefficients in \var{al}.
