Function: algsub
Section: algebras
C-Name: algsub
Prototype: GGG
Help: algsub(al,x,y): element x-y in al.
Doc: Given two elements $x$ and $y$ in \var{al}, computes their difference
 $x-y$ in the algebra \var{al}.
 \bprog
 ? A = alginit(nfinit(y), [-1,-1]);
 ? algsub(A,[1,1,0,0]~,[1,0,1,0]~)
 %2 = [0, 1, -1, 0]~
 @eprog

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