\begin{theindex}

 0\item a-delim0-1
  01\subitem ab-delim1-2
   1\subitem ac-delim1-3
   1\subitem ad-delim1-4
    12\subsubitem ae-delim2-5
     2\subsubitem af-delim2-6
     2\subsubitem ag-delim2-7

  \indexspace

 0\item b
  x1\subitem bb-delim1-1
   1\subitem bc-delim1-2
   1\subitem bd
    x2\subsubitem be-delim2-3
     2\subsubitem bf-delim2-4
     2\subsubitem bg-delim2-5

\end{theindex}