Method push

pbbslib/concurrent_stack.h:111–116

  size_t size() { return a.size(); }

  void push(T v) {
    Node* x = b.pop();
    if (!x) x = (Node*)malloc(sizeof(Node));
    x->value = v;
    a.push(x);
  }

  std::optional<T> pop() {
    Node* x = a.pop();