Function: bnfnarrow
Section: number_fields
C-Name: bnfnarrow
Prototype: G
Help: bnfnarrow(bnf): given a big number field as output by bnfinit, gives
 as a 3-component vector the structure of the narrow class group.
Doc: \var{bnf} being as output by
 \kbd{bnfinit}, computes the narrow class group of \var{bnf}. The output is
 a 3-component row vector $v$ analogous to the corresponding class group
 component \kbd{\var{bnf}.clgp}: the first component
 is the narrow class number \kbd{$v$.no}, the second component is a vector
 containing the SNF\sidx{Smith normal form} cyclic components \kbd{$v$.cyc} of
 the narrow class group, and the third is a vector giving the generators of
 the corresponding \kbd{$v$.gen} cyclic groups. Note that this function is a
 special case of \kbd{bnrinit}; the \var{bnf} need not contain fundamental
 units.
