library     SafeMath                    // This library is not used systematically since it tends to create "Infinite gas" functions and consumes too many gas
{
    //------------------
    function mul(uint256 a, uint256 b) internal pure returns (uint256) 
    {
        if (a == 0)     return 0;
        uint256 c = a * b;
        assert(c / a == b);
        return c;
    }
    //--------------------------------------------------------------------------
    function div(uint256 a, uint256 b) internal pure returns (uint256) 
    {
        return a/b;
    }
    //--------------------------------------------------------------------------
    function sub(uint256 a, uint256 b) internal pure returns (uint256) 
    {
        assert(b <= a);
        return a - b;
    }
    //--------------------------------------------------------------------------
    function add(uint256 a, uint256 b) internal pure returns (uint256) 
    {
        uint256 c = a + b;
        assert(c >= a);
        return c;
    }
}
////////////////////////////////////////////////////////////////////////////////
function sub(uint256 a, uint256 b) internal pure returns (uint256) 
{
    assert(b <= a);
    return a - b;
}

//--------------------------------------------------------------------------