Contract module that helps prevent reentrant calls to a function. Inheriting from ReentrancyGuard will make the {nonReentrant} modifier available, which can be applied to functions to make sure there are no nested (reentrant) calls to them. Note that because there is a single nonReentrant guard, functions marked as nonReentrant may not call one another. This can be worked around by making those functions private, and then adding external nonReentrant entry points to them. TIP: If you would like to learn more about reentrancy and alternative ways to protect against it, check out our blog post https://blog.openzeppelin.com/reentrancy-after-istanbul/[Reentrancy After Istanbul].