Function: alggroup
Section: algebras
C-Name: alggroup
Prototype: GDG
Help: alggroup(gal, {p=0}): constructs the group algebra of gal over Q (resp. Fp).
Doc: initializes the group algebra~$K[G]$ over~$K=\Q$ ($p$ omitted) or~$\F_p$
 where~$G$ is the underlying group of the \kbd{galoisinit} structure~\var{gal}.
 The input~\var{gal} is also allowed to be a \typ{VEC} of permutations that is
 closed under products.

 Example:
 \bprog
 ? K = nfsplitting(x^3-x+1);
 ? gal = galoisinit(K);
 ? al = alggroup(gal);
 ? algissemisimple(al)
 %4 = 1
 ? G = [Vecsmall([1,2,3]), Vecsmall([1,3,2])];
 ? al2 = alggroup(G, 2);
 ? algissemisimple(al2)
 %8 = 0
 @eprog
