void stpath_append_one (STPath& st, jtx::IOU const& iou) { st.push_back(STPathElement({ iou.account.id(), iou.currency, boost::none })); }
void stpath_append_one (STPath& st, jtx::BookSpec const& book) { st.push_back(STPathElement({ boost::none, book.currency, book.account })); }
void stpath_append_one (STPath& st, STPathElement const& pe) { st.push_back(pe); }