code: public void enableSrcPosCalculation(){
  		calculateSrcPositions = true;
  		final int len = sb.length();
  		ib = new ArrayList<Integer>(len);
  		for( int i=0; i<len; i++) ib.add( i );
  	}
docs: Enables the Calculation of Src Position. The base for these position
  will be the aktual, not the initial, String wich is uses as Base for
  the SpanManager.