Function: algsplittingmatrix
Section: algebras
C-Name: algsplittingmatrix
Prototype: GG
Help: algsplittingmatrix(al,x): image of x under a splitting of al.
Doc: A central simple algebra \var{al} output by \tet{alginit} contains data
 describing an isomorphism~$\phi : A\otimes_K L \to M_d(L)$, where $d$ is the
 degree of the algebra and $L$ is an extension of $L$ with~$[L:K]=d$. Returns
 the matrix $\phi(x)$.
 \bprog
 ? A = alginit(nfinit(y), [-1,-1]);
 ? algsplittingmatrix(A,[0,0,0,2]~)
 %2 =
 [Mod(x + 1, x^2 + 1) Mod(Mod(1, y)*x + Mod(-1, y), x^2 + 1)]

 [Mod(x + 1, x^2 + 1)                   Mod(-x + 1, x^2 + 1)]
 @eprog

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