@Override public Trie reduce(Reduce by){
  List<Trie> h=new ArrayList<>();
  for (  Trie trie : tries)   h.add(trie.reduce(by));
  MultiTrie2 m=new MultiTrie2(forward);
  m.tries=h;
  return m;
}

@ Override public Trie reduce ( Reduce by ) { List < Trie > h = new ArrayList < > ( ) ; for ( Trie trie : tries ) h . add ( trie . reduce ( by ) ) ; MultiTrie2 m = new MultiTrie2 ( forward ) ; m . tries = h ; return m ; }
remove empty rows from the given trie and return the newly reduced trie .
