15.40.txt

Exercise 15.40: In the OrQuery eval function what would happen if its rhs
member returned an empty set? What if its lhs member did so? What if both rhs
and lhs returned empty sets?

By Faisal Saadatmand

If rhs member returned an empty set:
Both right.begin() and right.begin() would return equal iterators, denoting a
range of zero. Consequently, the lines->insert operation would have no effect.

If lhs member returned an empty set:
ret_lines would be initialized to an empty set by make_shared, because both
begin and end iterators would be equal, and therefore, the call to make_shared
would be equivalent to 

	auto ret_lines = std::make_shared<std::set<line_no>>();

Since a set can grow dynamically, the operation that follows
(ret_lines->insert) would still be safe.

If both members returned an empty set:
The above two behaviours will occur and we end up with an empty ret_lines. In
other words, result of the OrQuery is that neither word (or a set or words) is
found in the text.