input: Write a function to add two matrices. The function should take two matrices as input and return their sum if they are compatible (i.e., they have the same dimension). If the matrices are not compatible, it should return a message indicating that the matrices are not compatible.
  
  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'
      
      rows = len(matrix1)
      cols = len(matrix1[0])
      result = [[sum(row) for row in zip(*size)] for size in zip(matrix1, matrix2)]
      
      return result
  
  assert add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]])==[[6, 8], [10, 12]]
  assert add_matrix([[1, 2, 3], [4, 5, 6]], [[7, 8, 9], [10, 11, 12]])==[[8, 10, 12], [14, 16, 18]]
  assert add_matrix([[1, 2], [3, 4]], [[1, 2, 3], [4, 5, 6]])=='Matrices not compatible'
  assert add_matrix([[1, 2, 3]], [[4, 5]])=='Matrices not compatible'
output: def add_matrix(matrix1, matrix2):
      if len(matrix1) != len(matrix2) or 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