d-of state S2
&gt; There is a friendship relation between S1 and S2
&gt; There is a constraint that the time period for S1 and S2 is the same.
&gt; 
&gt; So it boils down to spatio-temporal extents and timeless 
&gt; relations. You can
&gt; do this with all time based relations, though interestingly 
&gt; the analysis
&gt; varies for different types of relation, which is one the 
&gt; things that makes
&gt; it worthwhile.
&gt; Regards  
&gt; Matthew
&gt; ===========================================
&gt; Matt