Function: bnfisprincipal
Section: number_fields
C-Name: isprincipalall
Prototype: GGD1,L,
Help: bnfisprincipal(bnf,x,{flag=1}): bnf being output by bnfinit (with
 flag<=2), 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. flag is optional, whose meaning is: 0: output only v; 1: default; 2:
 output only v, precision being doubled until the result is obtained; 3: as 2
 but output generators

