assignment7 tweaks

[lambda.git] / hints / assignment_7_hint_6.mdwn

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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>
+
+