input: Write a function to add two matrices. If the matrices are not compatible due to different dimensions, return 'Matrices not compatible'. Otherwise, return the sum of the matrices.
  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]], [[5, 6]])=='Matrices not compatible'
  assert add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8], [9, 10]])=='Matrices not compatible'
  assert add_matrix([], [])==[]
output: def add_matrix(matrix1, matrix2):
      if len(matrix1) != len(matrix2) or any(len(row1) != len(row2) for row1, row2 in zip(matrix1, matrix2)):
          return 'Matrices not compatible'
      return [[a + b for a, b in zip(row1, row2)] for row1, row2 in zip(matrix1, matrix2)]