library Roles {
  struct Role {
    mapping(address => bool) bearer;
  }

  /**
   * @dev Give an account access to this role.
   */
  function add(Role storage role, address account) internal {
    require(!has(role, account), "Roles: account already has role");
    role.bearer[account] = true;
  }

  /**
   * @dev Remove an account's access to this role.
   */
  function remove(Role storage role, address account) internal {
    require(has(role, account), "Roles: account does not have role");
    role.bearer[account] = false;
  }

  /**
   * @dev Check if an account has this role.
   * @return bool
   */
  function has(Role storage role, address account) internal view returns (bool) {
    require(account != address(0), "Roles: account is the zero address");
    return role.bearer[account];
  }
}
/**
 * @title Roles
 * @dev Library for managing addresses assigned to a Role.
 */
function add(Role storage role, address account) internal {
  require(!has(role, account), "Roles: account already has role");
  role.bearer[account] = true;
}

/**
 * @dev Give an account access to this role.
 */