* @return the {@link SpeciesReference} of the {@link #listOfReactants}
* which has 'id' as species attribute (or name depending on the
* level and version). Can be null if it doesn't exist.
*/
public SpeciesReference getReactantForSpecies(String id) {
SpeciesReferenceFilter srf = new SpeciesReferenceFilter(id);
srf.setFilterForSpecies(true);
return getListOfReactants().firstHit(srf);
}