\begin{theindex}

  \item_0 a(0) 1
    \item_01 a(1) 2
      \item_12 a(2) 3
  \item_0 ab(0) 6
  \item_0 ad
    \item_x1 a(1) 8
    \item_1 b(1) 9
  \item_0 ae
    \item_x1 a
      \item_x2 a(2) 11
    \item_1 b
      \item_x2 b(2) 12

  \indexspace

  \item_0 pa(0) 21
    \item_01 qa
      \item_x2 ra(2) 22

  \indexspace

  \item_0 xa(0) 1
  \item_0 xb(0) 1
  \item_0 xc(0) 1

  \indexspace

  \item_0 ya
    \item_x1 x(1) 1
    \item_1 y(1) 1
    \item_1 z(1) 1
  \item_0 yb
    \item_x1 x
      \item_x2 x(2) 1
      \item_2 y(2) 1
      \item_2 z(2) 1

\end{theindex}