The issue is that the graph is not seen as weigthed by default. So either
you provide a weight function, or you have to set the graph has weigthed.
Then you get what you expect.
sage: G = Graph([(0,1,78), (0,2,99), (0,5,20), (1,3,16), (1,6,68), (1,7,34),
(2,4,43), (2,5,13), (3,4,70), (3,7,2),
Can you explain more fully what is wrong? The output looks like a spanning
tree to me. A question and a comment: what do you mean by "the result is
193"? Also, according to the documentation, the "starting_vertex" option is
only available for the algorithms "Prim_fringe" and "Prim_edge".
On
