input: Write a function to add two matrices. If the matrices are not of the same size, return 'Matrices not compatible'. Otherwise, return the sum of the two 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]], [[1, 2, 3], [4, 5, 6]])=='Matrices not compatible'
  assert add_matrix([[1, 2]], [[3], [4]])=='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))]