pan>$S$</span> where:</p><ul><li>the empty set, <span>$\varnothing$</span>, is an element of <span>$\mathcal{I}$</span> ,</li><li>if <span>$A \subseteq B \in \mathcal{I}$</span> then <span>$A \in \mathcal{I}$</span>, and</li><li>if <span>$A,B \in \mathcal{I}$</span> and <span>$|A| &lt; |B|$</span>, then there is an <span>$x \in B - A$</span> such that <span>$A \cup\{x\} \in \mathcal{I}$</span>. </li></ul><p>The set <span>$S$</span> is called the <em>ground set</em> of <span>$M$</span> and the sets in <span>