// int branch_number = traveller->parent()->which(traveller);
// if (branch_number == BRANCH_NOT_FOUND) non_continuable_error
// (class_name(), "generate_path", "branch not found during path construction");
// int branch_number = traveller->parent()->which(traveller);
// if (branch_number == BRANCH_NOT_FOUND) non_continuable_error
// (class_name(), "generate_path", "branch not found during path construction");