code: private List<IBond> heavyBonds(final List<IBond> bonds) {
          final List<IBond> heavy = new ArrayList<IBond>(bonds.size());
          for (final IBond bond : bonds) {
              if (!(bond.getBegin().getSymbol().equals("H") && bond.getEnd().getSymbol().equals("H"))) {
                  heavy.add(bond);
              }
          }
          return heavy;
      }
docs: Filter a bond list keeping only bonds between heavy atoms.
  
  @param bonds a list of bond
  @return the bond list only with heavy bonds