ts are wholly present
&gt; &gt; &gt; &gt; whenever it
&gt; &gt; &gt; &gt; &gt; &gt; &gt;is present.  I first strain to think of anything
&gt; &gt; that doesn't fit
&gt; &gt; &gt; &gt; &gt; &gt; &gt;this category.
&gt; &gt; &gt; &gt; &gt; &gt;
&gt; &gt; &gt; &gt; &gt; &gt; Well, an example might be a football match. It has a
&gt; &gt; &gt; &gt; first half and a
&gt; &gt; &gt; &gt; &gt; &gt; second half, and when one of them is present the other isn't.
&gt; &gt; &gt; &gt; &gt; &gt;
&gt; &gt; &gt; &gt; &gt; &g