code: def _edit2(self, w):
          
          # Of all spelling errors, 99% is covered by edit distance 2.
          # Only keep candidates that are actually known words (20% speedup).
          return set(e2 for e1 in self._edit1(w) for e2 in self._edit1(e1) if e2 in self)
docs: Returns a set of words with edit distance 2 from the given word