SolidityでMappingの入れ子をするには

SolidityでMapping in Mappingをする方法。

pragma solidity >=0.4.22 <0.6.0;

contract NestedMapping {

    // mapping(いつ => mapping(誰が => どれくらい)) ammountMapping;
    mapping(uint128 => mapping(address => uint128)) ammountMapping;

    function setAmmount(uint128 _date, uint128 _ammount) public {
        ammountMapping[_date][msg.sender] = _ammount;
    }

    function getAmmount(uint128 _date) view public returns(uint128) {
        return (ammountMapping[_date][msg.sender]);
    }
}