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+
+*	In def 3.1 on p. 14, GS&V define `s` updated with \[[not φ]] as:
+
+	>	{ i &elem; s | i does not subsist in s[φ] }
+
+	where `i` *subsists* in <code>s[&phi;]</code> if there are any `i'` that *extend* `i` in <code>s[&phi;]</code>.
+
+	Here's how we can represent that:
+
+		<pre><code>bind_set s (fun (r, h) ->
+			let u = unit_set (r, h)
+			in let descendents = u >>= \[[&phi;]]
+			in if descendents = empty_set then u else empty_set
+		</code></pre>
+
+