12. The method of claim 11 wherein said set-expression witness elements are selected from said first-level hash-buckets having an index higher than the log of said set-union cardinality estimate; and wherein said corresponding pair of first-level hash-buckets are singleton-union and singleton set-expression hash-buckets.
12. The method of claim 11 wherein said set-expression witness elements are selected from said first-level hash-buckets having an index higher than the log of said set-union cardinality estimate; and wherein said corresponding pair of first-level hash-buckets are singleton-union and singleton set-expression hash-buckets. 13. The method of claim 12 wherein said set-expression is one of a set-difference and a set-intersection.