input: Write a function to calculate the dot product of two lists. The function should return the dot product of the two input lists. If the two lists are not of the same length, the function should return an error message.
  assert dot_product([1, 2], [3, 4]) == 11
  assert dot_product([2, 3, 4], [4, 5, 6]) == 47
  assert dot_product([1, 2, 3], [3, 2, 1]) == 10
  assert dot_product([1], [1]) == 1
  assert dot_product([], []) == 0
  assert dot_product([1, 3], [2]) == "Error: Lists must be of equal length"
output: def dot_product(list1, list2):
      if len(list1) != len(list2):
          return "Error: Lists must be of equal length"
      return sum(x * y for x, y in zip(list1, list2))