Added a doctest

unnecessarymath.py

@@ -3,7 +3,15 @@
 import numpy as np
 
 def sqrt(x):
-    """Evaluate the square root of a non-negative number"""
+    """
+    Evaluate the square root of a non-negative number
+
+    >>> sqrt(4)
+    2.0
+
+    >>> sqrt(1.)
+    1.0
+    """
     return np.sqrt(x)
 
 def square(x):