- buy FOO with BAR:
	transaction.exchange cash->exchange BAR xxxx
	transaction.exchange exchange->cash FOO yyyy
- after trip, sell back all remaining FOO and then there should be no
  nonzero FOO totals anywhere but in exchange.
- in the presence of transactions between foreign currencies, system
  can be overspecified.  in this case, can break the loop by specifying
  a native equivalent amount for one transaction per extra equation;
  perhaps use average of exchange rates from transactions that are
  present in the db?  perhaps ask human?  perhaps try all ways of
  breaking the loop and see how much they differ?
- add an exchange category type, parallel to i/o/t in categories file,
  with weird semantics, for exchange transactions.

only relevant transactions (x flag):
initial purchases:
	cash: -100CDN, +500SEK; exch: +100CDN, -500SEK
	cash: -100CDN, +400DKK; exch: +100CDN, -400DKK
mid-trip:
	cash: -100SEK, +75DKK; exch: +100SEK, -75DKK
out.*: cash: -350SEK, 400DKK
	totals at this point: 50SEK, 75DKK
end:
	cash: -50SEK, +8CDN; exch: +50SEK, -8CDN
	cash: -75DKK, +15CDN; exch: +75DKK, -15CDN

totals:
	cash: CDN: -177 SEK: 0 DKK: 0
	exch: CDN: 177 SEK: -350 DKK: -400
	out: SEK: 350, DKK: 400

a * convSEK = 350
b * convDKK = 400
a + b = 177
x * convSEK = 100
x * convDKK = 75
----
a * convSEK = amt1
b * convDKK = amt2
c * convUSD = amt3
a + b + c = amt4
x1 * convSEK = amt5; x1 * convDKK = amt6
x2 * convSEK = amt7; x2 * convUSD = amt8
x3 * convDKK = amt9; x3 * convUSD = amt10
====
actual rates:
100CDN = 90USD, 500SEK, 400DKK
	CDN	USD	SEK	DKK	sell
CDN	---	1	.19	.23
USD	.85	---
SEK	5.75		---
DKK	4.5			---
buy
sell CDN: 100CDN buys 85USD, 575SEK, 450DKK
buy CDN: get 100CDN for 100USD, 550SEK, 450DKK

----
initial: sell 300CDN, get 90USD