[GAP Forum] IsSubgroup(FactorGroup(G/N),FactorGroup(H/N));

[lopo apelo kosho]

Dear friends;I would be very grateful if you advice/help m to handle a situation related to GAP: 
If N is a subgroup of H, H is subgroup of G, and N is normal in G; then H/N is subgroup of G/N. The problem is that GAP does not recognize it so. 
I have constructed the following code to check quotient closeness for some property, The code is: 
IsQClosedNSPSubgroups:=function(g) local pn,nsp,i,j,m;

if IsAbelian(g) then return true;

fi;

pn:=ProperNormalSubgroups(g);

if Size(NormalSubgroups(g))=Size(Subgroups(g)) thenreturn true;

fi;

nsp:=NSPSubgroups(g);

for i in [1..Size(pn)] do

m:=Filtered(nsp,x->IsSubgroup(x,pn[i]));

for j in [1..Size(m)] do 

if notIsNearlySPermutable(FactorGroup(g,pn[i]),FactorGroup(m[j],pn[i]))

then return false; 

fi;                                                                  

od;

od;

return true;

end;


THE ERROR MESSAGE IS:
Error, no method found! For debugging hints type ?Recovery from NoMethodFoundError, no 1st choice method found for `IsNormalOp' on 2 arguments called fromoper( super, sub ) at /proc/cygdrive/C/gap4r7/lib/domain.gd:439 called fromIsNormal( g, h ) at *stdin*:427 called fromIsNearlySPermutable( FactorGroup( g, pn[i] ), FactorGroup( m[j], pn[i] ) ) at *stdin*:685 called from<function "IsQClosedNSPSubgroups">( <arguments> ) called from read-eval loop at line 691 of *stdin*you can 'quit;' to quit to outer loop, oryou can 'return;' to continue

Thank you for your advice.