i,j:Vertices(the_graph). 
j = i 
i-the_graph- > *j i before j 
s) & (i,j,k:Vertices(the_graph). k-the_graph- > *j k-the_graph- > *i (k':Vertices(the_graph). k' before k L k'-the_graph- > *i) i before j s)
is mentioned
In prior sections: graph 1 3