code: def fts_contrast2(self, fs, ft_name, inv):
          
          inv_fts = [self.fts(x) for x in inv if set(fs) <= self.fts(x)]
          for a in inv_fts:
              for b in inv_fts:
                  if a != b:
                      diff = a ^ b
                      if len(diff) == 2:
                          if all([nm == ft_name for (_, nm) in diff]):
                              return True
          return False
docs: Return `True` if there is a segment in `inv` that contrasts in feature
          `ft_name`.
  
          Args:
              fs (list): feature specifications used to filter `inv`.
              ft_name (str): name of the feature where contrast must be present.
              inv (list): collection of segments represented as Unicode segments.
  
          Returns:
              bool: `True` if two segments in `inv` are identical in features except
                    for feature `ft_name`