Clarify comparison example between pure and impure functions

ch3.md

@@ -215,13 +215,31 @@ var signUp = function(attrs) {
   welcomeUser(user);
 };
 
+var saveUser = function(attrs) {
+    var user = Db.save(attrs);
+    ...
+};
+
+var welcomeUser = function(user) {
+    Email(user, ...);
+    ...
+};
+
 //pure
 var signUp = function(Db, Email, attrs) {
   return function() {
     var user = saveUser(Db, attrs);
     welcomeUser(Email, user);
   };
 };
+
+var saveUser = function(Db, attrs) {
+    ...
+};
+
+var welcomeUser = function(Email, user) {
+    ...
+};
 ```
 
 The example here demonstrates that the pure function must be honest about its dependencies and, as such, tell us exactly what it's up to. Just from its signature, we know that it will use a `Db`, `Email`, and `attrs` which should be telling to say the least.