Tax Table Tracking

[Rich Johnson]

I'm trying to get a handle on the TaxTable/BillTerm parent/child 
relationship; with an eye towards documenting it.  What follows comes 
from scanning the code and reading src/doc/business.txt.

Looking at just Invoice and TaxTable for now, it appears that the 
invariants for these objects should include:
- Posted invoice entries reference immutable child tax tables.
- Unposted invoices entries reference mutable parent tax tables.
- No more than one child tax table is created between modifications of 
the parent tax table.
- Parent/child tree is <=2 levels deep.  (no history of modification)

The child tax table invariant includes:
  - referenced by >=1 posted invoice entries
  - immutable
  - child field is always NULL (no further children)
  - refcount field is 0
  - invisible field is TRUE

The parent tax table invariant includes:
  - referenced by >=0  unposted invoice entries
  - mutable
  - parent field is always NULL (no further parents)
  - child field (when !NULL) references a child tax table with content 
identical to its parent.
  - child field NULL -> no referencing invoice entries posted since last 
tax table modification
  - child field !NULL -> at least one referencing invoice entry posted 
since last tax table modification
  - child field references most recently child, created  when the first 
referencing invoice entrychild and then only if an invoice has been 
posted since the last modification to the tax table.
  - refcount field contains the sum of the unposted invoice entries and 
the posted invoice entries which reference the child field.

- Modifying the TaxTable clears the child field and adjusts its 
refcount accordingly.
- Posting an invoice causes its entries to be modified to reference a 
child tax table; that child is created as needed.

FWIW, There are a lot of subtle implications to these invariants.  I 
have noticed that _unposting_ invoices does not seem to maintain them.  
I'm not sure whether or not that's a bug--or if the invariants need 
adjusting.

Comments?  Corrections?

Thanks,
--rich