Ex 4.3: add note about unique parameter names

info/exercises/src/ex-04/ex/semantics.tex

@@ -142,7 +142,8 @@
     \item Write the rules for the operational semantics for this language,
     assuming \emph{call-by-name} semantics for function calls. In call-by-name
     semantics, function arguments are not evaluated before the call. Instead,
-    the parameters are merely substituted into the function body.
+    the parameters are merely substituted into the function body. You may assume
+    that function parameters are named distinctly from variables in the program.
 
     \item Under the following environment (with function names, parameters, and
     bodies):