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export function removeItem<T>(arr: T[], item: T): T[] {  const index = arr.indexOf(item);  arr.splice(index, 1);  return arr;}
export function groupBy<T, U extends keyof T>(arr: T[], by: U): Map<T[U], T[]> {  const result = new Map<T[U], T[]>();  for (const item of arr) {    const key = item[by];    if (result.has(key)) result.get(key)!.push(item);    else result.set(key, [item]);  }  return result;}