Add comment

Dependencies

• [2] NULMDMUU some functions for 2021 day 12

Change contents

• edit in 2021/day12.rs at line 1
  [2.33]

  [2.34]
  + // I plan to use an unusual approach to this problem: it is not based on

  + // graph traversals but on transformations of directed graphs under which

  + // the number of paths bettween the start and end is invariant.