Initializable.sol

// SPDX-License-Identifier: MIT
pragma solidity >=0.6.0 <0.7.0;

/**
 * @dev This is a base contract to aid in writing upgradeable contracts, or any kind of contract that will be deployed
 * behind a proxy. Since proxied contracts do not make use of a constructor, it's common to move constructor logic to an
 * external initializer function, usually called `initialize`. It then becomes necessary to protect this initializer
 * function so it can only be called once. The {initializer} modifier provided by this contract will have this effect.
 *
 * TIP: To avoid leaving the proxy in an uninitialized state, the initializer function should be called as early as
 * possible by providing the encoded function call as the `_data` argument to {ERC1967Proxy-constructor}.
 *
 * CAUTION: When used with inheritance, manual care must be taken to not invoke a parent initializer twice, or to ensure
 * that all initializers are idempotent. This is not verified automatically as constructors are by Solidity.
 */
abstract contract Initializable {
  /**
   * @dev Indicates that the contract has been initialized.
   */
  bool private _initialized;

  /**
   * @dev Indicates that the contract is in the process of being initialized.
   */
  bool private _initializing;

  /**
   * @dev Modifier to protect an initializer function from being invoked twice.
   */
  modifier initializer() {
    // If the contract is initializing we ignore whether _initialized is set in order to support multiple
    // inheritance patterns, but we only do this in the context of a constructor, because in other contexts the
    // contract may have been reentered.
    require(_initializing ? _isConstructor() : !_initialized, "Initializable: contract is already initialized");

    bool isTopLevelCall = !_initializing;
    if (isTopLevelCall) {
      _initializing = true;
      _initialized = true;
    }

    _;

    if (isTopLevelCall) {
      _initializing = false;
    }
  }

  /**
   * @dev Modifier to protect an initialization function so that it can only be invoked by functions with the
   * {initializer} modifier, directly or indirectly.
   */
  modifier onlyInitializing() {
    require(_initializing, "Initializable: contract is not initializing");
    _;
  }

  /// @dev Returns true if and only if the function is running in the constructor
  function _isConstructor() private view returns (bool) {
    // extcodesize checks the size of the code stored in an address, and
    // address returns the current address. Since the code is still not
    // deployed when running a constructor, any checks on its code size will
    // yield zero, making it an effective way to detect if a contract is
    // under construction or not.
    address self = address(this);
    uint256 cs;
    assembly { cs := extcodesize(self) }
    return cs == 0;
  }

  // Reserved storage space to allow for layout changes in the future.
  uint256[50] private ______gap;
}