void swap(node-handle& nh)
  noexcept(ator_traits::propagate_on_container_swap::value ||
           ator_traits::is_always_equal::value);
Preconditions: 
!alloc_, or 
!nh.alloc_, or
ator_traits::propagate_on_container_swap::value is 
true,
or 
alloc_ == nh.alloc_. Effects: Calls 
swap(ptr_, nh.ptr_).  If 
!alloc_, or
!nh.alloc_, or 
ator_traits::propagate_on_container_swap::value
is 
true calls 
swap(alloc_, nh.alloc_).