By: Tom Sydney Kerckhove <syd@cs-syd.eu>
Add lot prices An asset held at a lot price becomes its own commodity: `2 SWDA lot @ 500 EUR` is an amount of 2 in the commodity "SWDA acquired at 500 EUR per unit", a distinct key in every balance map from plain `SWDA` and from `SWDA` bought at another price. A lot balances at its lot rate, exactly like `@` does, and converts one to one into the commodity it is a lot of, so it is worth whatever the underlying is worth rather than what it cost. Disposal is a negative posting in the same lot, which leaves the difference between basis and proceeds for a capital gains posting to absorb. A lot-free assertion sums across every lot of a symbol, so an assertion copied off a broker statement stays writable once lots are in play. This also splits `Currency`, which was doing two jobs: a declared currency, and whatever a balance is denominated in. The lot rode on it as a Maybe field, which is why `declaredCurrency` had to exist. A `Commodity` is now a currency or a lot, and one rule decides every site: conversion targets are currencies, balance keys are commodities. What follows from the types rather than from a validity rule: * a cost converts to a `Currency`, so nothing can convert into a lot * a `Lot` holds a `Currency`, so a lot of a lot cannot be built * `PostingPrice` has nowhere to put a cost beside a lot A currency can no longer be called `lot`.