Transaction voiding

[Bill Gribble]

I'm interested in adding support for transaction voiding to the engine.
That is, the transaction and splits continue to exist in the engine but
they don't affect balances.  

It's possible to fake this with a reversing transaction, but in general
that's not really accurate; there would be an period between the initial
transaction and the reversing one where account balances would be
incorrect, where a true "void" just says "this transaction never
happened, but we want to continue to keep a record of its entry". 

Any thoughts about how to do this?  It would be possible to add voiding
splits to the transaction (i.e. an equal-and-opposite split for each
original split, in the same transaction) plus a note in the KVP
structure for the transaction saying that the transaction is voided ...
but is that correct from an accounting perspective?  Should we really be
just marking the transaction (or its splits) as void and somehow
explicitly ignoring them in the engine? 

b.g.