code: private ReactionRole role(IAtom atom) {
          ReactionRole role = atom.getProperty(CDKConstants.REACTION_ROLE);
          if (role != null)
              return role;
          return ReactionRole.None;
      }
docs: Safely access the reaction role of an atom, returns {@link ReactionRole#None} if null.
  
  @param atom atom
  @return mapidx, None if undefined