doc/concepts.tex: Fix directed-graph terminology.

Vertex/edge are for undirected graphs; node/arc are for directed ones.

diff --git a/doc/concepts.tex b/doc/concepts.tex
index b452f3c..528a551 100644 (file)
--- a/doc/concepts.tex
+++ b/doc/concepts.tex
@@ -123,10 +123,10 @@ If an object is a direct instance of class~$C$ then the object is also an
 (indirect) instance of every superclass of $C$.
 
 If $C$ has a proper superclass $B$, then $B$ must not have $C$ as a direct
-superclass.  In different terms, if we construct a graph, whose vertices are
-classes, and draw an edge from each class to each of its direct superclasses,
-then this graph must be acyclic.  In yet other terms, the `is a superclass
-of' relation is a partial order on classes.
+superclass.  In different terms, if we construct a directed graph, whose
+nodes are classes, and draw an arc from each class to each of its direct
+superclasses, then this graph must be acyclic.  In yet other terms, the `is a
+superclass of' relation is a partial order on classes.
 
 \subsubsection{The class precedence list}
 This partial order is not quite sufficient for our purposes.  For each class