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(account != address(0), "address cannot be 0x");
        require(!has(role, account), "account already in role");

        role.bearer[account] = true;
    }

    /**
     * @dev remove an account's access to this role
     */
    function remove(Role storage role, address account) internal {
        require(account != address(0), "address cannot be 0x");
        require(has(role, account), "account not in 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), "address cannot be 0x");
        return role.bearer[account];
    }
}
/**
 * @title Roles
 * @dev Library for managing addresses assigned to a Role.
 */
function add(Role storage role, address account) internal {
        require(account != address(0), "address cannot be 0x");
        require(!has(role, account), "account already in role");

        role.bearer[account] = true;
    }
/**
 * @dev give an account access to this role
 */