. If the first input informs the union operator
that all tuples with attribute A in the range [5,15] have been read,
it would be incorrect for the union operator to simply pass that
punctuation through. It is possible for the second input to still
have values for A in that same range.
Instead, the union operator must store the punctuation. If the next
punctuation from the second input is for the range [10,20], then the
operator can intersect the two ranges, and pass on punctuation for
the range [10,15] for attribute A. If the next punctuation from the
second input is for the range [20,25], then the union operator can
intersect this range with the range from its first input. In this
case, [5,15] Ç [20,25] is the empty
set, so no punctuation is passed.

*Last modified by Pete Tucker on 26 August 2005.*