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	(
	    data_size => $#data,     # 0 .. $#data.
	    page_size => $page_size, # 1 .. N.
	);

	my(@bound) = $page -> offset(- 1) -> bounds();

the call to C<sub offset(- 1)> will have no effect.

That is, when trying to go back past the beginning of the data set, the
bounds will be locked to values within 0 .. data_size.

Similarly, a call which would go beyond the other end of the data set,
will lock the bounds to the same range.

In short, you can't fall off the edge by calling C<sub offset()>.

This in turn means that the values returned by C<sub bounds()> will
always be valid indexes within the range 0 .. data_size.

The module implements this by building 2 objects of type Set::Window,