==> logic/river.crossing.s <==
The three columns represent the left bank, the boat, and the right bank
respectively. The < or > indicates the direction of motion of the boat.

HHHMmm	.	.
HHHm	Mm>	.
HHHm	<M	m
HHH	Mm>	m
HHH	<M	mm
HM	HH>	mm
HM	<Hm	Hm
Hm	HM>	Hm
Hm	<Hm	HM
mm	HH>	HM
mm	<M	HHH
m	Mm>	HHH
m	<M	HHHm
.	Mm>	HHHm
.	.	HHHMmm