code: public static MarkedElement markupBond(IRenderingElement elem, IBond bond) {
          assert elem != null;
          MarkedElement tagElem = markupChemObj(elem, bond);
          tagElem.aggClass("bond");
          return tagElem;
      }
docs: Markup a bond with the class 'bond' and optionally the ids/classes
  from it's properties.
  
  @param elem rendering element
  @param bond bond
  @return the marked element