diff --git a/hazardflow-designs/src/examples/fir_filter.rs b/hazardflow-designs/src/examples/fir_filter.rs
index 4598cfc..11bd8fe 100644
--- a/hazardflow-designs/src/examples/fir_filter.rs
+++ b/hazardflow-designs/src/examples/fir_filter.rs
@@ -4,33 +4,33 @@ use crate::prelude::*;
 use crate::std::*;
 
 impl<P: Copy + Default> Valid<P> {
-    /// Window combinator
+    /// Window combinator.
+    ///
     /// It takes a stream of input value P and return the latest N values.
     fn window<const N: usize>(self) -> Valid<Array<P, N>>
-    where [(); N + 1]: {
-        self.fsm_map(P::default().repeat::<N>(), |ip, s| {
-            let ep = s.append(ip.repeat::<1>()).clip_const::<N>(0);
-            let s_next = ep;
+    where
+        [(); N - 1]:,
+        [(); 1 + (N - 1)]:,
+    {
+        self.fsm_map(P::default().repeat::<{ N - 1 }>(), |ip, s| {
+            let ep = ip.repeat::<1>().append(s).resize::<N>();
+            let s_next = ep.clip_const::<{ N - 1 }>(0);
             (ep, s_next)
         })
     }
 }
 
 impl<const N: usize> Valid<Array<u32, N>> {
-    /// Weight combinator
-    fn weight(self, weight: [u32; N]) -> Valid<Array<u32, N>> {
-        self.map(|ip| ip.zip(Array::from(weight)).map(|(ele, weight)| ele * weight))
-    }
-
-    /// Sum combinator
-    /// It will add up all the elements within an array.
+    /// Sum combinator.
+    ///
+    /// It adds up all the elements within an array.
     fn sum(self) -> Valid<u32> {
-        self.map(|ip| ip.fold_assoc(|e1, e2| e1 + e2))
+        self.map(|ip| ip.fold(0, |acc, e| acc + e))
     }
 }
 
 /// FIR filter implementation
 #[synthesize]
 pub fn fir_filter(input: Valid<u32>) -> Valid<u32> {
-    input.window::<3>().weight([4, 2, 3]).sum()
+    input.window::<3>().map(|ip| ip.zip(Array::from([4, 2, 3])).map(|(e, wt)| e * wt)).sum()
 }