, 
(partition:
mutually exclusive and exhaustive), and for all 
, 
and 
.
This problem is naturally expressed as a bipartite graph.
Such a graph G=(V,E) has a set of nodes L and a set of nodes
R such that , (partition:
mutually exclusive and exhaustive), and for all , and .
Also natural for machines-tasks, classes-classrooms, boys-girls, senders-receivers, servers-clients, clothes-bodyparts, meat-wine, etc.
