Function: galoissubcyclo
Section: number_fields
C-Name: galoissubcyclo
Prototype: GDGD0,L,Dn
Help: galoissubcyclo(N,H,{fl=0},{v}):Compute a polynomial (in variable v)
 defining the subfield of Q(zeta_n) fixed by the subgroup H of (Z/nZ)*. N can
 be an integer n, znstar(n) or bnrinit(bnfinit(y),[n,[1]],1). H can be given
 by a generator, a set of generator given by a vector or a HNF matrix (see
 manual). If flag is 1, output only the conductor of the abelian extension.
 If flag is 2 output [pol,f] where pol is the polynomial and f the conductor.

