Function: bnrisprincipal
Section: number_fields
C-Name: bnrisprincipal
Prototype: GGD1,L,
Help: bnrisprincipal(bnr,x,{flag=1}): bnr being output by bnrinit, gives
 [v,alpha], where v is the vector of exponents on the class group
 generators and alpha is the generator of the resulting principal ideal. In
 particular x is principal if and only if v is the zero vector. If (optional)
 flag is set to 0, output only v

