make [x/y] use 'define' uniformly

Author: pfps

spec/index.html

@@ -274,7 +274,7 @@ <h2>Notation and Terminology</h2>
     <li>Otherwise, t[x/y] = t.</li>
   </ol>
   <p>
-    For each triple (s, p, o), we set (s, p, o)[x/y] = (s[x/y], p[x/y], o[x/y]). For each graph G, we obtain G[x/y] by applying [x/y] to each triple t in G.
+    For any triple (s, p, o), we define (s, p, o)[x/y] as (s[x/y], p[x/y], o[x/y]). For any graph G, we define G[x/y] as the result of applying [x/y] to each triple t in G.
   </p>