Investigate `Giving up, Schreier tree is not shallow` messages and systematically deal with them

[fingolfin]

These messages come from orb, and can be turned off via

SetInfoLevel(InfoOrb, 0);

In fact in PR #406 I inserted that into the classical naming code to avoid annoying warnings being printed when naming SO(+1,4,4):

        # avoid warnings 'Giving up, Schreier tree is not shallow' (in general
        # one shouldn't ignore this kind of warning but here we know which
        # groups are input, and it is acceptable)
        ol := InfoLevel(InfoOrb);
        SetInfoLevel(InfoOrb, 0);
        ord := RECOG.EstimateProjOrder(grp);
        SetInfoLevel(InfoOrb, ol);
        if ord mod 3600 <> 0 then  # FIXME: Giving up, Schreier tree is not shallow

(dang, that FIXME comment was left in by accident)

Anyway: in that particular case I verified that the warning is harmless and didn't see another good way to avoid it.

The same technique can be used in other cases. But it should be done with care: sometime it might hide real issues, e.g. a stabilizer chain being used when it shouldn't be.

Therefore, if anyone sees these warnings, please leave a comment indicating where and how, esp. if you then proceed to disable it with the above technique.

[TWiedemann]

Happens to me about 1 out of 5 times with SL(5,2).

gap> RecogniseGroup(SL(5,2));
#I  Giving up, Schreier tree is not shallow.
F dim=   5 field=2  0    
<recognition node GoProjective Dim=5 Field=2
 F:<recognition node (projective) ClassicalNatural Comment=SL(5,2)_StabilizerChain Size=9999360 Dim=5 Field=2>
 K:<trivial kernel>

However, the recognition node still seems to work.

gap> IsReady(ri);
true
gap> Size(ri);
9999360

[fingolfin]

@TWiedemann in your case, setting SetInfoLevel(InfoRecog, 2);SetInfoLevel(InfoMethSel, 2); reveals a clue:

#I  Classical natural: SL(5,2): small case, handing over to Schreier-Sims.
#I  Giving up, Schreier tree is not shallow.

So this seems to be a case where we deliberately use a stabilizer chain. Some more context of the code issuing that message:

  if d = 2 then
      ...
  else   # bigger than 2:
      if classical.isSLContained = true then
          # Do not run the generic code in small cases:
          if (q^d-1)/(q-1) <= 1000 or d = 3 then
              # FIXME: Note d=3 currently has a problem in the SL2-finder.
              Info(InfoRecog,2,"Classical natural: SL(",d,",",q,"): small ",
                   "case, handing over to Schreier-Sims.");
              ri!.comment := Concatenation("SL(",String(d),",",String(q),")",
                                           "_StabilizerChain");
              return FindHomMethodsProjective.StabilizerChainProj(ri);
          fi;

I think it would be fair to disable InfoOrb here.

Since we may end up doing this a lot, perhaps we should have a special helper... Say

RunWithoutOrb := function(f)
    local old, ret;
    old := InfoLevel(InfoOrb);
    SetInfoLevel(InfoOrb, 0);
    ret := f();
    SetInfoLevel(InfoOrb, old);
    return ret;
end;

And then we can replace

              return FindHomMethodsProjective.StabilizerChainProj(ri);

by

              return RunWithoutOrb({} -> FindHomMethodsProjective.StabilizerChainProj(ri));