code: func WithStemmer(s lingo.Stemmer) TrainerConsOpt {
  	f := func(t *Trainer) {
  		// cannot pass in itself
  		if T, ok := s.(*Trainer); ok && T == t {
  			panic("Recursive setting of stemmer! (Trying to set t.stemmer = T)")
  		}
  		t.s = s
  	}
  	return f
  }
docs: // WithStemmer sets up the stemmer option on the DependencyParser