itemitem

<..plain+ itemitem list..>
 \def\:temp#1{\:iilist \let\:iilist=\empty
    \def\EnditemitemList{\end:iilist \let\EnditemitemList=\empty}%
    \a:iitem {#1}\b:iitem}
 \HLet\itemitem=\:temp
 \long\def\c:itemitem:#1#2#3#4{
    \let\EnditemitemList=\empty
    \c:def\:iilist{#1}%
    \c:def\end:iilist{#2\c:def\:iilist{#1}}
    \c:def\a:iitem{#3}
    \c:def\b:iitem{#4}}
 -_-_-

If the second parameter is not empty, it is up to the user to introduce \EnditemitemList.