SOURCE

// const a = sum(0); // => a === 0
// const b = sum(1)(2); // => b === 3
// const c = sum(4)(5); // c === 9
// const k = sum(n1)...(nk) // k === n1 + n2 + ... + nk
function sum(){
    //实现代码
}

const a = sum(0); // => a === 0
const b = sum(1)(2); // => b === 3
const c = sum(4)(5); // c === 9