Mention that lists can be stored in sets after freezing
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MOS_MIGRATED_REVID=135472917
diff --git a/site/versions/master/docs/skylark/concepts.md b/site/versions/master/docs/skylark/concepts.md
index 782db50..dc749c4 100644
--- a/site/versions/master/docs/skylark/concepts.md
+++ b/site/versions/master/docs/skylark/concepts.md
@@ -115,7 +115,8 @@
`BUILD` file is loaded in a different context. Each rule is also analyzed in a
separate context. We allow side-effects (e.g. appending a value to a list or
deleting an entry in a dictionary) only on objects created during the current
-evaluation context.
+evaluation context. Once the code in that context is done executing, all of its
+values are frozen.
For example, here is the content of the file `foo.bzl`:
@@ -161,6 +162,9 @@
* Sets have reference equality semantics and can be stored in other sets.
+* Lists and other mutable values may be stored in sets and in dictionary
+ keys once they are frozen.
+
* Modifying a collection during iteration is an error. You can avoid the error
by iterating over a copy of the collection, e.g.
`for x in list(my_list): ...`. You can still modify its deep contents
