Exercise: accumulator

Write a function func makeAccumulator() (func(int) int, func()). It returns two functions that share one sum. The first is an accumulator: it adds its argument to the sum and returns the new sum. The second resets the sum to 0. The sum of each accumulator starts at 0.

• add, reset := makeAccumulator(), then add(5) → 5
• the next call add(3) → 8
• after reset(), the call add(4) → 4

Constraints: the arguments are from -1000 to 1000 and may be negative or zero; each pair of functions created by a separate call to makeAccumulator has its own sum, and calls to one pair do not change another.

Do not add extra imports unless the task says so explicitly.