[$e_{1}$,$e_{2}$]$\sim$([$a$,$b$].$p$($a$;$b$))+ $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$[$e_{1}$;$e_{2}$]$\sim$([$a$,$b$].$p$($a$;$b$))$\ast$[$a$,$b$].$p$($a$;$b$)