input: Write a function to add two matrices. The function should check if the dimensions of the matrices are compatible for addition. If they are, it should return the resulting matrix after addition. If the dimensions are not compatible, it should return 'Matrices not compatible'.
  
  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]])=='Matrices not compatible'
  assert add_matrix([[1, 2, 3]], [[4, 5, 6]])==[[5, 7, 9]]
  assert add_matrix([[1]], [[2]])==[[3]]
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))]