set of sets
<math xmlns="http://www.w3.org/1998/Math/MathML">
<set>
<set>
<ci>a</ci>
<ci>d</ci>
</set>
<set>
<ci>x</ci>
<ci>y</ci>
<ci>z</ci>
</set>
<set>
<cn>0</cn>
<cn>2</cn>
<cn>6</cn>
<cn>8</cn>
<cn>9</cn>
</set>
</set>
</math>
Author: Design Science, Inc. (E. Cannon, E. Tabacman, R.Miner)