0.4.2.3 Euler Cycles
A cycle in a graph is a traversal which visits no node more
than once. An Euler cycle is a special cycle that traverses
all the edges in a graph and visits every node at least once.
It can be proven that an Euler cycle can exist in a graph if
and only if all vertices are of even degree. That is to say,
the must be an even number of edges emanating from every
vertex in the graph.
