input: Write a function to add two matrices. The function should return the sum of the matrices if they are compatible for addition, otherwise it should return a message indicating that the matrices are not compatible.
  assert add_matrix([[1,2],[3,4]], [[3,4],[5,6]])==[[4,6],[8,10]]
  assert add_matrix([[1,2],[3,4]], [[3,4],[5,6],[7,8]])=='Matrices not compatible'
  assert add_matrix([[1,2],[3,4]], [[3,4]])=='Matrices not compatible'
  assert add_matrix([[1,2],[3,4]], [[3,4,5],[5,6,7]])=='Matrices not compatible'
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 [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]