code: public Span adjust(int offset, int n){
  		if( offset < 0 )return this; //null
  		
  		if( offset < end ){
  			end += n;
  			if( end<offset )end = offset;
  		}
  		else return this; //null
  		
  		if( offset < start ){
  			start += n;
  			if( start<offset )start = offset;
  		}
  		return this;	
  	}
docs: Adjusts the start and end Position of the Span, if they are
  larger than the offset.