To obfuscate ITEM 21 <a href=http://www.inwap.com/pdp10/hbaker/hakmem/boolean.html#item21>here</a>:
Given a subset S of the subsets of X, find the smallest subset of S whose union is X.
<p>Using more jargon we can recast the problem: “Given a covering of X, find the smallest sub-covering.”.