bool isOrderNeeded() const noexcept
indicates whether a new complete ordering is needed
const std::vector< NodeId > * order() const noexcept
returns the current complete ordering
const std::vector< NodeId > * _order_
the vector indicating in which order we should eliminate the nodes
bool _order_needed_
indicate whether a new complete ordering is necessary for the elimination
gum is the global namespace for all aGrUM entities