/**
 \addtogroup Business
 @{
    \addtogroup ImmutableTaxTableBillTerm TaxTable and BillTerm immutability tracking

Derek Atkins <warlord@mit.edu>
Version of October 2003

The gncTaxTable and gncBillTerm business objects have parent, child,
refcount, invisible field in their structures that deserve some
explanation:

- a child is a 'frozen' instance of a parent.  For example, the tax
  percentage in a particular tax table may change over time, but you
  dont want that change to affect already-posted invoices...  So you
  make sure there is an immutable 'copy' (read: child) of the tax
  table when you post the invoice and repoint at the child.
                                                                                
- a parent can have many children, but it will only have a 'child'
  pointer if the parent has not been modified.  Think of this as a
  copy-on-write mechanism.  posted invoices will continue to use the
  _same_ child until the parent is modified, at which point a new
  child will be created.
                                                                                
- invisible means "dont show this in the list".  It's so you dont
  get all the children in the tax table list -- you only see parents.
  I suppose this flag could also be called "is-child" as I believe that
  only children can be invisible, and ALL children are invisible.
                                                                                
- refcount is a listing of how many objects are referencing it.
  Basically, it's letting you know how many customer, vendor, entries,
  etc are referencing e.g. a particular tax table object.  mostly this
  was done to make sure you cannot delete an in-use taxtable.
                                                                                
- children don't use refcounts, only parents do.

- A child always points to its parent (it can have only 1).
- A parent has a list of all children.
- A parent has a pointer to the current 'replica child', if one exists.

*/