diff --git a/Nim-for-Python-Programmers.md b/Nim-for-Python-Programmers.md
index d70bcd4..ca1163b 100644
--- a/Nim-for-Python-Programmers.md
+++ b/Nim-for-Python-Programmers.md
@@ -188,7 +188,7 @@ echo lc[ y*2 | ( y <- x ), int ]
 Python sets are not like [Nim set type](http://nim-lang.org/docs/manual.html#types-set-type).
 If the values that will go in the set are known beforehand and finite, you can create an Enum for them.
 Otherwise you can emulate a Python set using a [HashSet](http://nim-lang.org/docs/sets.html).
-The Nim set type is faster and memory-efficient.
+The Nim set type is faster and memory-efficient. In fact, set is implemented with a bit vector, whereas HashSet is implemented as a dictionary. For simple flag types and small mathematical sets, use set.
 
 ### Dictionaries