diff --git a/PRELUDE.md b/PRELUDE.md
index 0166af6..55fc238 100644
--- a/PRELUDE.md
+++ b/PRELUDE.md
@@ -30,4 +30,17 @@ Use: `(pixels erase-color (255 0 0))`.
         (eq a:0 b:0) 
         (eq a:1 b:1) 
         (eq a:2 b:2)) 0 255)))
+```
+
+### Posterize
+
+Use: `(pixels posterize 40)`.
+
+```
+(defn posterize 
+  (a q) 
+  (
+    (step a:0 q) 
+    (step a:1 q) 
+    (step a:2 q) a:3))
 ```
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