void add (GraphVertex from, GraphVertex to, bool via_sends_only);
bool has (GraphVertex from, GraphVertex to, bool* via_sends_only);
std::set<GraphVertex> from (GraphVertex r) const;
void add (GraphVertex from, GraphVertex to, bool via_sends_only);
bool has (GraphVertex from, GraphVertex to, bool* via_sends_only);
std::set<GraphVertex> from (GraphVertex r) const;