code: public static IReaction getRelevantReaction(IChemModel chemModel, IAtom atom) {
          IReaction reaction = null;
          if (chemModel.getReactionSet() != null) {
              IReactionSet reactionSet = chemModel.getReactionSet();
              reaction = ReactionSetManipulator.getRelevantReaction(reactionSet, atom);
          }
          return reaction;
      }
docs: Retrieves the first IReaction containing a given IAtom from an
  IChemModel.
  
  @param chemModel  The IChemModel object.
  @param atom       The IAtom object to search.
  @return           The IAtomContainer object found, null if none is found.