input: Write a python function to generate all possible combinations of a given set of numbers.
  assert combinations([1, 2, 3]) == [[1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1]]
  assert combinations([4, 5]) == [[4, 5], [5, 4]]
  assert combinations([]) == [[]]
output: def combinations(numbers):
      from itertools import permutations
      return list(map(list, permutations(numbers)))