input: Write a function to add two matrices. The function should return the sum of the two matrices if they are of the same dimensions. If the matrices are not compatible for addition, it should return "Matrices 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 = [[matrix1[i][j] + matrix2[i][j] for j in range(cols)] for i in range(rows)]
      
      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]])=='Matrices not compatible'
  assert add_matrix([[1, 2], [3, 4]], [[5, 6]])=='Matrices not compatible'
  assert add_matrix([], [])==[]
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'
      
      rows = len(matrix1)
      cols = len(matrix1[0])
      result = [[matrix1[i][j] + matrix2[i][j] for j in range(cols)] for i in range(rows)]
      
      return result