diff --git a/ChangeLog b/ChangeLog
index 34eab0841d..36d1c0ce6b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Aug  3 00:11:08 2011  Tanaka Akira  <akr@fsij.org>
+
+	* lib/prettyprint.rb: update document.  [ruby-core:36776]
+
+
 Tue Aug  2 20:10:16 2011  Shota Fukumori  <sorah@tubusu.net>
 
 	* test/testunit/test_parallel.rb: pass "--ruby" option to
diff --git a/lib/prettyprint.rb b/lib/prettyprint.rb
index 8b33bdf85f..9a90713a4d 100644
--- a/lib/prettyprint.rb
+++ b/lib/prettyprint.rb
@@ -157,7 +157,14 @@ class PrettyPrint
     end
   end
 
-  # This is similar to #breakable, but is less-likely to insert a newline.
+  # This is similar to #breakable except
+  # the decision to break or not is determined individually.
+  #
+  # Two #fill_breakable under a group may cause 4 results:
+  # (break,break), (break,non-break), (non-break,break), (non-break,non-break).
+  # This is different to #breakable because two #breakable under a group
+  # may cause 2 results:
+  # (break,break), (non-break,non-break).
   #
   # The text sep+ is inserted if a line is not broken at this point.
   #