"unknown" and "not_applicable"

[Leonard Soicher]

Dear All,

I suggest that we need to think more clearly about when we 
can and should use "unkown" and "not_applicable" as attribute 
values. 

For example, consider the the indicator <resolvable>.  This has an
attribute called `value' which may be boolean, "unkown" or
"not_applicable".  By stating "unkown" are we making a specific point
that we don't know (and need help)?  If so, we may also want to allow
value="unknown" for the indicator <partially_balanced> or the
automorphism group property <multiplicity_free>.  Or should we just
leave out certain optional tags when we don't know the answer (which
may be difficult to compute).

Now what does "not_applicable" mean for resolvable? Either a block
design is resolvable or it isn't. A t-(v,k,lambda) with v not divisible
by k is *not* resolvable; we should not use "not_applicable" in this
case.  We should be precise mathematically *and* explain what we are
doing and why we have made the choices we have. Similarly, with
natural and appropriate definitions, an intransitive permutation group
is *not* primitive, *not* generously transitive and *not*
multiplicity-free. (I forget what stratifiable means.) Permutation rank
is naturally defined only for transitive groups, so for this property,
the option "not_aplicable" makes sense.

Regards,   Leonard