CHG: Bring empty function into namespace for tdd

- otherwise assertions fail with NameError

Changes to be committed:
	modified:   _episodes/10-defensive.md

Author: tensionhead

_episodes/10-defensive.md

@@ -304,7 +304,15 @@ Its advocates believe it produces better code faster because:
     rather than to find errors.
 2.  Writing tests helps programmers figure out what the function is actually supposed to do.
 
-Here are three test functions for `range_overlap`:
+We start by defining an empty function `range_overlap`:
+
+~~~
+def range_overlap(ranges):
+    pass
+~~~
+{: .language-python}
+
+Here are three test statements for `range_overlap`:
 
 ~~~
 assert range_overlap([ (0.0, 1.0) ]) == (0.0, 1.0)
@@ -326,10 +334,9 @@ AssertionError:
 {: .error}
 
 The error is actually reassuring:
-we haven't written `range_overlap` yet,
-so if the tests passed,
-it would be a sign that someone else had
-and that we were accidentally using their function.
+we haven't implemented any logic into `range_overlap` yet,
+so if the tests passed, it would indicate that we've written
+an entirely ineffective test.
 
 And as a bonus of writing these tests,
 we've implicitly defined what our input and output look like: