Add a subtract() method to collections.Counter()
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index 176975b..aa17a55 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -214,6 +214,17 @@
>>> Counter('abracadabra').most_common(3)
[('a', 5), ('r', 2), ('b', 2)]
+ .. method:: subtract([iterable-or-mapping])
+
+ Elements are subtracted from an *iterable* or from another *mapping*
+ (or counter). Like :meth:`dict.update` but subtracts counts instead
+ of replacing them. Both inputs and outputs may be zero or negative.
+
+ >>> c = Counter(a=4, b=2, c=0, d=-2)
+ >>> d = Counter(a=1, b=2, c=3, d=4)
+ >>> c.subtract(d)
+ Counter({'a': 3, 'b': 0, 'c': -3, 'd': -6})
+
The usual dictionary methods are available for :class:`Counter` objects
except for two which work differently for counters.