input: Write a function to calculate the sum of all elements in a list of lists.
  assert sum_of_element([[1, 2], [3, 4], [5, 6]]) == 21
  assert sum_of_element([[0, 0], [0, 0], [0, 0]]) == 0
  assert sum_of_element([[-1, -2], [-3, -4], [-5, -6]]) == -21
output: def sum_of_element(list_of_lists):
      return sum(sum(inner_list) for inner_list in list_of_lists)