ENHANCE: New method for IntermediateSubgroups using maximal subgroups

lib/grplatt.gi

@@ -2306,9 +2306,70 @@ local rt,op,a,l,i,j,u,max,subs;
   return rec(subgroups:=List(a,i->ClosureGroup(U,rt{i})),inclusions:=max);
 end);
 
+InstallMethod(IntermediateSubgroups,"blocks for coset operation",
+  IsIdenticalObj, [IsGroup,IsGroup],
+  1, # better than previous if index larger
+function(G,U)
+local uind,subs,incl,i,j,k,m,gens,t,c,p;
+  if (not IsFinite(G)) and Index(G,U)=infinity then
+    TryNextMethod();
+  fi;
+  uind:=IndexNC(G,U);
+  if uind<200 then
+    TryNextMethod();
+  fi;
+  subs:=[G]; #subgroups so far
+  incl:=[];
+  i:=1;
+  gens:=SmallGeneratingSet(U);
+  while i<=Length(subs) do
+    # find all maximals containing U
+    m:=MaximalSubgroupClassReps(subs[i]);
+    m:=Filtered(m,x->IndexNC(subs[i],U) mod IndexNC(subs[i],x)=0);
+    Info(InfoLattice,1,"Subgroup ",i,", Order ",Size(subs[i]),": ",Length(m),
+      " maxes");
+    for j in m do
+      t:=RightTransversal(subs[i],Normalizer(subs[i],j)); # conjugates
+      for k in t do
+        if ForAll(gens,x->k*x/k in j) then
+	  # U is contained in j^k
+	  c:=j^k;
+	  Assert(1,IsSubset(c,U));
+	  #is it U?
+	  if uind=IndexNC(G,c) then
+	    Add(incl,[0,i]);
+	  else
+	    # is it new?
+	    p:=PositionProperty(subs,x->IndexNC(G,x)=IndexNC(G,c) and
+	      ForAll(GeneratorsOfGroup(c),y->y in x));
+	    if p<>fail then 
+	      Add(incl,[p,i]);
+	    else
+	      Add(subs,c);
+	      Add(incl,[Length(subs),i]);
+	    fi;
+	  fi;
+	fi;
+      od;
+    od;
+    i:=i+1;
+  od;
+  # rearrange
+  c:=List(subs,x->IndexNC(x,U));
+  p:=Sortex(c);
+  subs:=Permuted(subs,p);
+  subs:=subs{[1..Length(subs)-1]}; # remove whole group
+  for i in incl do
+    if i[1]>0 then i[1]:=i[1]^p; fi;
+    if i[2]>0 then i[2]:=i[2]^p; fi;
+  od;
+  Sort(incl);
+  return rec(inclusions:=incl,subgroups:=subs);
+end);
+
 InstallMethod(IntermediateSubgroups,"normal case",
   IsIdenticalObj, [IsGroup,IsGroup],
-  1,# better than the previous method
+  2,# better than the previous methods
 function(G,N)
 local hom,F,cl,cls,lcl,sub,sel,unsel,i,j,rmNonMax;
   if not IsNormal(G,N) then