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 . Indices . Length && this . Flags [ this . End + 1 ] ) { ++ this . End ; } } else {