diff --git a/pymbolic/mapper/__init__.py b/pymbolic/mapper/__init__.py
index 1d7c1e9d4ba78b151d737d71296e080a6b6e6495..ebbe98072ef4789d9158ef4fa8ef3c098dc2413d 100644
--- a/pymbolic/mapper/__init__.py
+++ b/pymbolic/mapper/__init__.py
@@ -307,6 +307,8 @@ class Collector(CombineMapper):
     across children at each non-leaf node in the expression tree.
 
     By default, nothing is collected. All leaves return empty sets.
+
+    .. versionadded:: 2014.3
     """
 
     def combine(self, values):