Let v be a set. Suppose v the edges of G . For every chain p of G holds p Seg n is a chain of G . Suppose G 1 G . Then the source of G 1 the source of G and the target of G 1 the target of G .
G is a sum of G and G .
Then G 1 G 1 G 2 and G 2 G 1 G 2 . G 1 G and G 2 G , then G 1 G 1 G 2 and G 2 G 1 G 2 . If G 1 G and G 2 G , then G 1 G 2 G . Then G 1 G 2 . For every oriented graph G such that G 1 G holds G 1 is oriented.