diff --git a/patacrep/index.py b/patacrep/index.py
index 346c1acd..c29b88ab 100644
--- a/patacrep/index.py
+++ b/patacrep/index.py
@@ -77,6 +77,7 @@ class Index(object):
 
     def add_keyword(self, key, word):
         """Add 'word' to self.keywords[key]."""
+        # Because LaTeX uses 'sep'
         if key == 'sep':
             key = 'separators'
         if key not in self.keywords: