# Our list of lists.
matrix_1 = [[1,1,1], [2,2,2], [3,3,3]]
matrix_2 = [[4,4,4], [5,5,5], [6,6,6]]

# Matrix addition with for loop.
# Assuming that the matrices are of the same dimensions
matrix_sum = []
for row in range(len(matrix_1)):
  matrix_sum.append([])
  for column in range(len(matrix[0])):
      matrix_sum[row].append(matrix_1[row][column] + matrix_2[row][column])

# Rewrite using list comprehension only for inner lists.
matrix_sum = []
for row in range(len(matrix_1)):
  matrix_sum.append([matrix_1[row][column] + matrix_2[row][column] for column in range(len(matrix[0]))])

# Rewrite using nested list comprehension.
matrix_sum = [[matrix_1[row][column] + matrix_2[row][column] for column in range(len(matrix[0]))]
              for row in range(len(matrix_1))]

print(matrix_sum) # Output: [[5, 5, 5], [7, 7, 7], [9, 9, 9]]