Function: rnfisnorm
Section: number_fields
C-Name: rnfisnorm
Prototype: GGD0,L,
Help: rnfisnorm(T,x,{flag=0}): T is as output by rnfisnorminit applied to
 L/K. Tries to tell whether x is a norm from L/K. Returns a vector [a,b]
 where x=Norm(a)*b. Looks for a solution which is a S-integer, with S a list
 of places in K containing the ramified primes, generators of the class group
 of ext, as well as those primes dividing x. If L/K is Galois, omit flag,
 otherwise it is used to add more places to S: all the places above the
 primes p <= flag (resp. p | flag) if flag > 0 (resp. flag < 0). The answer
 is guaranteed (i.e x norm iff b=1) if L/K is Galois or, under GRH, if S
 contains all primes less than 12.log(disc(M))^2, where M is the normal
 closure of L/K

