export function testOnce(): number[] {
  function once<T>(fn: () => T): () => T {
    let flag = false
    let res: T;
    return () => {
      if(!flag) {
      flag = true
      res = fn()
      }
      return res
    }
  }

  let count = 0;
  const increment = once(() => ++count);
  return [increment(), increment(), increment()];
}