input: Write a python function to merge two lists into one.
  assert merge([1, 2, 3], [4, 5, 6]) == [1, 2, 3, 4, 5, 6]
  assert merge(['a', 'b', 'c'], ['d', 'e', 'f']) == ['a', 'b', 'c', 'd', 'e', 'f']
  assert merge([1, 3, 5], [2, 4, 6]) == [1, 3, 5, 2, 4, 6]
output: def merge(list1, list2):
      return list1 + list2