`T` has a fixed point, then there exists some `X` such that `X <~~>
TX` (that's what it means to *have* a fixed point).
- let W = \x.T(xx) in
- let X = WW in
- X = WW = (\x.T(xx))W = T(WW) = TX
+<pre>
+let W = \x.T(xx) in
+let X = WW in
+X = WW = (\x.T(xx))W = T(WW) = TX
+</pre>
Q: How do you know that for any term T, YT is a fixed point of T?