8a7ecb43

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Add lot types to the ledger

Work in progress: this commit does not compile. It is committed so the work
can be picked up on another machine; LOT-PRICES-HANDOVER.md section 6.4 lists
the exact edits that finish it.

A currency gains an optional lot, so that a lot is a distinct key in every
balance map and lot balances never mix. Lot holds the acquisition rate and
its currency inline rather than nesting a Currency, which makes a lot of a lot
unrepresentable and, more importantly, keeps a currency location-stable: the
only annotations inside it are declaration locations, so two mentions of the
same lot in different places compare equal.

The build breaks at the four currency-membership checks in the Ledger validity
instance, which is the undefined trick and the partial application of the
Currency constructor doing their job.

LOT-PRICES-HANDOVER.md carries the whole design: the agreed syntax and why it
beat the alternatives, the balancing and valuation rules, the phase plan with
a green criterion per phase, the state of every file, and the facts about this
codebase that were expensive to work out.

Timeline