diff --git a/README.md b/README.md
index e16dc12..fe7d154 100644
--- a/README.md
+++ b/README.md
@@ -49,6 +49,17 @@ Thus, care must be taken when serializing structures containing function.
 ### `partialDeepCopy(..)`
 
 
-
+## Format
+
+The output of `.serialize(..)` is a strict superset of standard JSON, 
+while the input format is a bit more relaxed than JSON in several details.
+
+Extensions to JSON:
+- Recursion
+- null types
+- BigInt
+- Map / Set
+- Function as code
+- Function as index