private SB builder(){
  assert indent > 0;
  SB sb=new SB();
  for (int i=0; i < indent; i++)   sb.a(TAB);
  return sb;
}

private SB builder ( ) { assert indent > 0 ; SB sb = new SB ( ) ; for ( int i = 0 ; i < indent ; i ++ ) sb . a ( TAB ) ; return sb ; }
creates new builder with correct indent .
