/// \param style Dot style of the edge (e.g., "dotted" or "solid").
/// \param fout output stream.
void output_edge (int source, int destination, char const *label,
/// \param style Dot style of the edge (e.g., "dotted" or "solid").
/// \param fout output stream.
void output_edge (int source, int destination, char const *label,