input: Write a function to add two matrices. If the matrices have different dimensions, return 'Matrices not compatible'. If the matrices are empty, return an empty matrix.
  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([], [])==[]
  assert add_matrix([[1,2],[3,4]], [[1,2,3],[4,5,6]])=='Matrices not compatible'
output: def add_matrix(matrix1, matrix2):
      if len(matrix1) != len(matrix2):
          return 'Matrices not compatible'
      if len(matrix1) == 0:
          return []
      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