diff --git a/3or4/4.org b/3or4/4.org
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@@ -21,6 +21,11 @@ Prove that the following functions is not $\chi$ computable (where /Exp/ is the
 You should prove this by reducing the "intensional" halting problem (see the [[https://chalmers.instructure.com/courses/36941/file_contents/course%20files/lectures/L4.pdf][lecture slides]]) to this function.
 
 ** Answer
+The halting problem can be reduced to this problem, by the expression:
+\[ \lambda p. f\ ((\lambda \_. Zero()) p) \]
+
+In the case where p terminates, the $(\lambda \_. Zero()) p$ would evaluate to $Zero()$, and therefore $f ((\lambda \_. Zero()) p)$ would evaluate to true.
+In the case where p does not terminate, then $(\lambda \_. Zero()) p$ would not evaluate to $Zero()$ as $p$ would not terminate with a value, therefore $f ((\lambda \_. Zero()) p)$ would evaluate to false.
 
 * (2p)
 Implement $\chi$ substitution as a $\chi$ program. You should construct a $\chi$ expressions, let us call it /subst/, that given the representation of a variable $x$, a closed expression $e$, and an expression $e'$, produces the representation of the expression obtained by substituting $e$ for every free occurrence of $x$ in $e'$: