Half a convoy looked exactly like a whole one

Multi-fleet convoys work and always did -- London to Spain across the
Channel and the Mid-Atlantic with both fleets carrying, which is what the
fifty DATC convoy cases are about. What did not work was finding that out
from inside the game.

A convoy takes as many orders as there are seas to cross, and the rules give
two different answers to writing fewer. With no fleet ordered at all the
move is illegal -- legality is decided on the board alone -- and the panel
reddens it. With one fleet of a two-fleet chain the move is perfectly legal
and simply fails, silently, a turn later, with nothing on screen to tell it
apart from having been blocked.

From the chair where the orders are written those are one complaint: nobody
is carrying this. And it is worse than it looks, because the board
deliberately offers crossings that run through other powers' fleets, which
cannot be ordered at all, only asked for -- so a player can write a crossing
that no order of theirs could ever complete.

The panel now asks the question that matters rather than the one the rules
happen to answer: given everything on the table, is there a chain that gets
this army there? When there is not it says which kind of not, in amber
rather than the red an illegal order gets, because a legal order that will
fail is a different thing to tell somebody.

README.md

@@ -333,6 +333,27 @@ sits there in red until `F North Sea convoys Yorkshire → Norway` joins it,
 and then both go black. Nothing was added to make that happen. The validator
 already knew.
 
+### Half a convoy
+
+Multi-fleet convoys work -- London to Spain across the Channel and the
+Mid-Atlantic, with both fleets ordered to carry -- and the DATC's fifty
+convoy cases say so. What did not work was finding that out.
+
+A convoy takes as many orders as there are seas to cross, and the rules give
+two different answers to writing fewer. No fleet ordered at all and the move
+is *illegal*: legality is decided on the board alone, so the panel reddens
+it. One fleet of a two-fleet chain and the move is perfectly *legal*. It
+simply fails, silently, a turn later, with nothing on screen to distinguish
+it from having been blocked.
+
+From the chair where the orders are written those are the same complaint --
+nobody is carrying this. And it is worse than it looks, because the board
+deliberately offers crossings that run through other powers' fleets, which
+cannot be ordered at all, only asked for. So the panel now answers the one
+question that matters: given everything on the table, is there a chain that
+gets this army there? If not it says which is missing -- *no fleet has been
+ordered to carry it*, or *ordered to carry it, but not the whole way*.
+
 ## Before the map, and how to stop
 
 A landing page, because Diplomacy has a reputation and about half of it is