input: Write a python function to find all unique combinations of two elements from a given list.
  assert combinations([1, 2, 3]) == [[1, 2], [1, 3], [2, 3]]
  assert combinations([4, 5, 6, 7]) == [[4, 5], [4, 6], [4, 7], [5, 6], [5, 7], [6, 7]]
  assert combinations([10, 20]) == [[10, 20]]
output: def combinations(lst):
      result = []
      for i in range(len(lst)):
          for j in range(i + 1, len(lst)):
              result.append([lst[i], lst[j]])
      return result