AgGroupOps.Stabilizer( U, pt, op )
Let U be an ag group acting on a set Omega by op. Let pt be an
element of Omega. Then AgGroupOps.Stabilizer returns the stabilizer
of pt in U.
op must be a function taking two arguments such that op( p, u ) is
the image of a point p in Omega under the action of an element u of
U. If conjugation should be used op must be OnPoints.
The stabilizer is computed by stepping up the composition series of U.
The whole orbit <pt>^<U> is not stored during the computation (see
LNS84). Of course this saving of space is bought at the cost of
time. If you need a faster function, which may use more memory, you can
use AgOrbitStabilizer (see AgOrbitStabilizer) instead.
GAP 3.4.4