code: public LineSegment subSegment(double offset, double length) {
          Point subSegmentStart = pointAlongLineSegment(offset);
          Point subSegmentEnd = pointAlongLineSegment(offset + length);
          return new LineSegment(subSegmentStart, subSegmentEnd);
      }
docs: LineSegment that starts at offset from start and runs for length towards end point
  
  @param offset offset applied at begin of line
  @param length length of the new segment
  @return new LineSegment computed