like:
<dl>
<dt>sloop
-<dd>The capacity of a sloop, exactly
+<dd>The capacity of a sloop, leaving no allowance for rum and shot
+<dt>sloop - 1%
+<dd>The capacity of a sloop minus 1%
<dt>20t 13kl
-<dd>20 tonnes (20,000kg), 13 kilolitres (13,000l)
+<dd>13 tonnes (13,000kg), 20 kilolitres (20,000l)
<dt>sloop - 100l 100kg
<dd>The capacity of a sloop minus 100l, minus 100kg
<dt>2t plus 500kg minus 200kg
<dd>2300kg, with no limit on volume
</dl>
+Evaluation is strictly from left to right.
+
+<p>
Formally, the capacity is a list of terms, all but the first preceded
by one of <kbd>-</kbd>, <kbd>minus</kbd>, <kbd>+</kbd>,
<kbd>plus</kbd>. Each term may specify a mass and/or a volume
(separated by a space), as a number followed (without an intervening
space) by a unit (<kbd>t</kbd>, <kbd>kg</kbd>, <kbd>kl</kbd> or
-<kbd>l</kbd>). The first term may be a ship name or abbrevation
-instead. If the first term specifies only one of mass or volume, all
-the subsequent terms may only adjust that same value.
+<kbd>l</kbd>). Alternatively each term except the first may specify a
+percentage, which is applied as a percentage change to the answer from
+all the preceding terms. The first term may be a ship name or
+abbrevation instead. If the first term specifies only one of mass or
+volume, all the subsequent terms may only adjust that same value.
<h3>Expected losses</h3>