Excerise: Implement graph difference.

Exercise: Implement graph union.

Write a union method for a graph so that, for $G_1=(V_1,E_1)$ and $G_2=(V_2,E_2),$ with $G_1$ represented by $g1$ in code and $G_2$ represented by g$2,$

g$1|$ g$2$

returns the graph $G_{12}=(V_1{V}_2,E_1{E}_2)$ having as vertices the union of the vertices of $G_1$ and $G_2,$ and as edges the union of the edges of $G_1$ and $G_2.$

$$ Returning the edges

In our latest implementation, we do not have direct access to the edges of the graph. In other words, for a graph g$,$ we cannot do:

for $(u,v)$ in g$.$edges:

We ask you to write an iterator over edges, to make the above code work. The iterator should yield the edges of the graph, one by one.