design doc: debits and credits

[Michael Leary]

I hate to nit, and I realize that everyone surely knows this ... but ... I was
just looking at one of the design docs ( xacc/src/doc/design/engine.texinfo )
where it says:

"Positive Split values are 'debits' and negative Split values are 'credits'.
Forcing the Splits to 'add up' to zero causes a double-entry accounting system
to always balance."

No.  Debit or credit has nothing to do with whether something is positive or
negative -- in fact a negative split value would most likely point to
confusion about double entry bookkeeping...  Debits are on the left side,
credits are on the right side of an account; and that's all you can say about
them.  All asset and expense accounts are *increased* on the debit side, while
all liability, revenue, and capital accounts are *decreased* on the debit
side.  The only thing that has to "add up" is that debits must equal credits
for any single transaction.

Maybe add a diagram like this diagram to the design doc (it always helped me
keep it all straight) ...

    A     =       L       +          OE
 -----------------------------------------------
          |
 Checking |  Credit Card     Retained Earnings
 -------- |  -----------     -----------------
  + | -   |   -   |  +          -    |   +
    |     |       |                  |
          |
          |                  Expense   Revenue
          |                  -------   -------
          |                   + | -     - | +
          |                     |         |