[GAP Forum] ClosureGroup!

[lopo apelo kosho]

Dear friends, 
I am writing a GAP code to check the following property: The join of any two permutable subgroup is a PERMUTABLE subgroup. I have a code that gives all permutable subgroups in a given group (PermutableSubgroups(g)). Here is my code:


IsPermJoinGroup:=function(g)local i,n,j,c;

if IsAbelian(g)=truethen 

return true;

fi; 

n:=PermutableSubgroups(g);

if Size(n)=1 thenreturn true;

fi;

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

for j in [i+1..n]do

c:= ClosureGroup (n[i],n[j]);

if not c in n   then 

return false; 

fi;

od;

od;

return true;

end;
When I checked if  s4:= the symmetric group on 4 elements satisfies this property I get this message:

IsPermJoinGroup(s4);
Error, Range: <last> must be an integer (not a list (plain,hom)) in
  for j in [ i + 1 .. n ] do
    c := ClosureGroup( n[i], n[j] );
    if not c in n then
        return false;
    fi;
od; at *stdin*:1313 called from
<function "IsPermJoinGroup">( <arguments> )
 called from read-eval loop at *stdin*:1319
you can replace <last> via 'return <last>;'

Any suggestion please. 
Thank you.