\begin{definition}[Definition:River Crossing Problem]
A '''river crossing problem''' concerns a group of people, who may or may not have a collection of objects, which need to cross a river.
The constraints of the problem are generally of the form:
:$(1): \quad$ The boat can carry only a limited cargo, thereby making it impossible for the crossing to be made in one go
:$(2): \quad$ Certain combinations of people or objects or both may not be left alone or in combination on either bank of the river.
In some versions of the problem, there are one or more islands midstream at which people or objects may be left for a while.
\end{definition}