[The following letter accidently got lost. Reply follows. mS]

Is the function `Normalizer(A,B)' , with `A' and `B' in a common
parent group, supposed to work even if `B' is not a subgroup of `A'?
If yes, then the following seems to provide a bug:

gap> g:=Group((1,6,13,5,4,2,15,10,14,12,3,9,7,11,8),(1,16)(2,3)(4,5)(6,7)(8,9)(10,11)(12,13)(14,15),(1,3)(4,8)(5,11)(6,10)(7,9)(13,15));;
gap> k:=Subgroup(g,[(1,2)(3,16)(4,7)(5,6)(8,11)(9,10)(12,15)(13,14),(1,3)(2,16)(4,6)(5,7)(8,10)(9,11)(12,14)(13,15)]);;
gap> u:=Stabilizer(g,1);;
gap> nu1:=Normalizer(u,k);;
gap> nu2:=Intersection(u,Normalizer(g,k));;
gap> Index(nu2,nu1);
3
gap> quit;

Best wishes,
              Peter