input: Write a function to add two matrices. If the dimensions of the matrices do not match, return 'Matrices not compatible'. Otherwise, return a new matrix that is the sum of the two input matrices.
  assert add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]]) == [[6, 8], [10, 12]]
  assert add_matrix([[1, 2]], [[5, 6]]) == [[6, 8]]
  assert add_matrix([[1, 2]], [[5]]) == 'Matrices not compatible'
  assert add_matrix([[1], [3]], [[5], [6]]) == '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))]