Only one true in the tract
if ( this -> Start == this -> End ) { this -> Flags [ this -> End ] = false ; this -> Flags [ this -> End + 1 ] = true ; this -> Start += 1 ; this -> End += 1 ; while ( this -> End + 1 < this -> N && this -> Flags [ this -> End + 1 ] ) { ++ this -> End ; } } else {