\centerline{ \structure{\nopositioncheck\wmostaromatatom{C}\nsingle\atom{C} \nesingle\nmostaromatatom{C}\sesingle\emostaromatatom{C}\ssingle \atom{C}\swsingle\smostaromatatom{C}\nwsingle\phantatom{C}} \hskip 1truecm \structure{\nopositioncheck\atom{C}\ndouble\atom{C}\nesingle \firstbicycloatom{C}\sesingle\atom{C}\sdouble\atom{C}\swsingle \secondbicycloatom{C}\nwsingle\phantatom{C}} \hskip 1truecm \structure{\nopositioncheck\firstbicycloatom{C}\nsingle \phantatom{C}\nesingle\secondbicycloatom{C}\swphantom \firstbicycloatom{C}\nephantom\phantatom{C}\sesingle\atom{C} \sdouble\atom{C}\swsingle\secondbicycloatom{C}\nwsingle \phantatom{C}} \hskip 1truecm \structure{\nopositioncheck\firstbicycloatom{C}\sesingle \phantatom{C}\nephantom\secondbicycloatom{C}\swsingle \firstbicycloatom{C}\nephantom\phantatom{C}\nsingle\phantatom{C} \nwsingle\secondbicycloatom{C}\sephantom\firstbicycloatom{C} \nwphantom\phantatom{C}\swsingle\secondbicycloatom{C}\ssingle \phantatom{C}} } \medskip\centerline{\bf Figure~3. \rm Four different benzene structures.}