define DNERule (#33)

oxinabox · ararslan · commit a2e6451bd8d7 · 2019-05-29T11:51:29.000-07:00
diff --git a/src/rules.jl b/src/rules.jl
@@ -152,7 +152,11 @@ end
 #####
 
 """
-TODO
+    DNERule(args...)
+
+Construct a `DNERule` object, which is an `AbstractRule` that signifies that the
+current function is not differentiable with respect to a particular parameter.
+**DNE** is an abbreviation for Does Not Exist.
 """
 struct DNERule <: AbstractRule end
 
