This feature turns out to be quite involved, as valuation interacts
with the many report variations. Various bugs/specs have been
fixed/clarified relating to register's running total, balance totals
etc. Eg register's total should now be the sum of the posting amount
values, not the values of the original sums. Current level of support
has been documented.
When valuing at transaction date, we once again do early valuation of
all posting amounts, to get more correct results. variants. This means
--value-at=t can be slower than other valuation modes when there are
many transactions and many prices. This could be revisited for
optimisation when things are more settled.