diff --git a/js/src/Y.js b/js/src/Y.js
--- a/js/src/Y.js
+++ b/js/src/Y.js
@@ -6,9 +6,11 @@
 
 // Return the function that is the fixed point of f.
 const Y = f => (x => f(v => x(x)(v)))
                (x => f(v => x(x)(v)));
 
+const  Z = f  => (f());
+
 // The factorial function is the fixed point of this:
 const f = fac => n => (n <= 1) ? 1 : n * fac(n - 1);
 
 print(`5! is ${Y(f)(5)}`);