input: Write a function to add two matrices. If the dimensions of the matrices are not compatible for addition, return the string 'Matrices not compatible'. Otherwise, return the sum of the two matrices.
  
  def add_matrix(matrix1, matrix2):
      if len(matrix1) != len(matrix2):
          return 'Matrices not compatible'
      if len(matrix1[0]) != len(matrix2[0]):
          return 'Matrices not compatible'
      
      result = [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]
      
      return result
  
  assert add_matrix([[1, 2], [3, 4]], [[1, 2], [3, 4]]) == [[2, 4], [6, 8]]
  assert add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]]) == [[6, 8], [10, 12]]
  assert add_matrix([[1, 2, 3], [4, 5, 6]], [[1, 2, 3], [4, 5, 6]]) == [[2, 4, 6], [8, 10, 12]]
  assert add_matrix([[1, 2], [3, 4]], [[1, 2, 3], [4, 5, 6]]) == 'Matrices not compatible'
  assert add_matrix([[1, 2], [3, 4]], [[1, 2]]) == 'Matrices not compatible'
output: def add_matrix(matrix1, matrix2):
      if len(matrix1) != len(matrix2):
          return 'Matrices not compatible'
      if len(matrix1[0]) != len(matrix2[0]):
          return 'Matrices not compatible'
      
      result = [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]
      
      return result