Function: algsub
Section: algebras
C-Name: algsub
Prototype: DGGG
Help: algsub({al},x,y): element x-y in al (Hamilton quaternions if omitted).
Doc: Given two elements $x$ and $y$ in \var{al} (Hamilton quaternions if
 omitted), 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}.

 If~$x$ and~$y$ are given in the same format, then one should simply use \kbd{-}
 instead of \kbd{algsub}.
