input: Write a function to add two matrices. If the dimensions of the matrices are not the same, return 'Matrices not compatible'. If the input matrices are not empty and are compatible, 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]])==[[4, 6]]
  assert add_matrix([[1],[2],[3]], [[4],[5],[6]])==[[5],[7],[9]]
  assert add_matrix([[1,2],[3,4]], [[5,6]])=='Matrices not compatible'
  assert add_matrix([], [])=='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))]