Function: bnfisnorm
Section: number_fields
C-Name: bnfisnorm
Prototype: GGD1,L,p
Help: bnfisnorm(bnf,x,{flag=1}): Tries to tell whether x (in Q) is the norm
 of some fractional y (in bnf). Returns a vector [a,b] where x=Norm(a)*b.
 Looks for a solution which is a S-unit, with S a certain list of primes (in
 bnf) containing (among others) all primes dividing x. If bnf is known to be
 Galois, set flag=0 (in this case, x is a norm iff b=1). If flag is non zero
 the program adds to S all the primes : dividing flag if flag<0, or less than
 flag if flag>0. The answer is guaranteed (i.e x norm iff b=1) under GRH, if
 S contains all primes less than 12.log(disc(Bnf))^2, where Bnf is the Galois
 closure of bnf

