Function: polcompositum
Section: number_fields
C-Name: polcompositum0
Prototype: GGD0,L,
Help: polcompositum(pol1,pol2,{flag=0}): vector of all possible compositums
 of the number fields defined by the polynomials pol1 and pol2. If (optional)
 flag is set (i.e non-null), output for each compositum, not only the
 compositum polynomial pol, but a vector [pol,al1,al2,k] where al1 (resp.
 al2) is a root of pol1 (resp. pol2) expressed as a polynomial modulo pol,
 and a small integer k such that al2+k*al1 is the chosen root of pol

