[GAP Forum] some questions

[R. Keith Dennis]

Dear Jack, 
Dear Steve,

Thanks again for the helpful replies.

> > I'm too accustomed to programming in perl where I protect myself
> > (most of the time) by "if defined ... then".
> > 
> We are working on this. Already, many functions return 'fail' rather than
> giving errors (for instance trying to invert a singular matrix) to allow these
> conditions to be detected, but we are working on a general mechanism for
> catching errors. 

Ok, I was not aware that there were so many differences in the
programming (e.g., I noted about 1000 instances of 'fail') and had
preseumed that the programs all followed, more or less, the same
standards.

> I'll leave your remaining question, about normal subgroups for someone else to
> answer.

No suggestions yet.

I hope there is still the patience to answer a few more questions:

If one wanted to write a function, SubLattice say, that might behave
in some reasonable way, e.g., maybe something like

  l:=LatticeSubgroups(G);

  n:=SubLattice(l,x->NormalSubgroups(G));

where SubLattice is supposed to remove all nodes and edges from l
except for those in the collection of subgroups given (at the moment I
won't worry if the resulting structure is actually a lattice), one at
least needs to understand the way the lattice structure is stored in
l.  How do I determine that?  IsRecord, IsRecordCollection,
IsRecordCollColl all returned false.  Display doesn't help.  Is there
a list of 'types' of objects used in GAP?  If so, I somehow missed it.
[I guess the code in xgap might give some hints, but I haven't had a
chance to look at that carefully yet.]

And a related question, if for example, I wanted to determine how
RankMat worked, say, I wondered if (and I'm sure it does) immediately
returned n as the rank for an m x n matix for m > n once enough
independent rows are found, how would I go about tracing backwards
through the code to find this?

I'm sure these sorts of things have been asked before & there's
probably some place I should have already looked, but I didn't 
manage to find it.

I hope I'm not being too much of a pain.

Thanks again for all your help!

Keith