input: Write a function to add two matrices. If the dimensions of the two matrices do not match, return 'Matrices not compatible'. Otherwise, return the sum of the two matrices.
  def add_matrix(matrix1, matrix2):
      if len(matrix1) != len(matrix2):
          return 'Matrices not compatible'
      if len(matrix1[0]) != len(matrix2[0]):
          return 'Matrices not compatible'
      
      rows = len(matrix1)
      cols = len(matrix1[0])
      result = [[sum(row) for row in zip(*size)] for size in zip(matrix1, matrix2)]
      
      return result
  
  assert add_matrix([[1,2],[3,4]], [[5,6],[7,8]])==[[6,8],[10,12]]
  assert add_matrix([[1,2],[3,4]], [[1,2,3],[3,4,5]])=='Matrices not compatible'
  assert add_matrix([[1,2],[3,4]], [[1,2],[3,4]])==[[2,4],[6,8]]
  assert add_matrix([[1,2,3],[4,5,6]], [[7,8,9],[10,11,12]])==[[8,10,12],[14,16,18]]
output: def add_matrix(matrix1, matrix2):
      if len(matrix1) != len(matrix2) or len(matrix1[0]) != len(matrix2[0]):
          return 'Matrices not compatible'
      
      result = [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]
      
      return result