diff --git a/pep-0387.txt b/pep-0387.txt
index 89179612d..aa65adbca 100644
--- a/pep-0387.txt
+++ b/pep-0387.txt
@@ -37,6 +37,10 @@ This policy applies to all public APIs.  These include:
 
 - Function, class, module, attribute, and method names and types.
 
+- Given a set of arguments, the return value, side effects, and raised
+  exceptions of a function.  This does not preclude changes from
+  reasonable bug fixes.
+
 - The position and expected types of arguments and returned values.
 
 - Behavior of classes with regards to subclasses: the conditions under