Function: rnfnormgroup
Section: number_fields
C-Name: rnfnormgroup
Prototype: GG
Help: rnfnormgroup(bnr,pol): norm group (or Artin or Takagi group)
 corresponding to the Abelian extension of bnr.bnf defined by pol, where
 the module corresponding to bnr is assumed to be a multiple of the
 conductor. The result is the HNF defining the norm group on the
 generators in bnr.gen.
Doc:
 \var{bnr} being a big ray
 class field as output by \kbd{bnrinit} and \var{pol} a relative polynomial
 defining an \idx{Abelian extension}, computes the norm group (alias Artin
 or Takagi group) corresponding to the Abelian extension of
 $\var{bnf}=$\kbd{bnr.bnf}
 defined by \var{pol}, where the module corresponding to \var{bnr} is assumed
 to be a multiple of the conductor (i.e.~\var{pol} defines a subextension of
 bnr). The result is the HNF defining the norm group on the given generators
 of \kbd{bnr.gen}. Note that neither the fact that \var{pol} defines an
 Abelian extension nor the fact that the module is a multiple of the conductor
 is checked. The result is undefined if the assumption is not correct,
 but the function will return the empty matrix \kbd{[;]} if it detects a
 problem; it may also not detect the problem and return a wrong result.
