code: static boolean isPlainBond(IBond bond) {
          return bond.getOrder() == IBond.Order.SINGLE
                  && (bond.getStereo() == IBond.Stereo.NONE || bond.getStereo() == null);
      }
docs: A plain bond is a non-stereo sigma bond that is displayed simply as a line.
  
  @param bond a non-null bond
  @return the bond is plain