diff --git a/output/src/ops.rs b/output/src/ops.rs
index 4c6ea2a..7eba655 100644
--- a/output/src/ops.rs
+++ b/output/src/ops.rs
@@ -2,6 +2,7 @@ mod align;     pub use self::align::*;
 mod bsp;       pub use self::bsp::*;
 mod cond;      pub use self::cond::*;
 mod map;       pub use self::map::*;
+mod memo;      pub use self::memo::*;
 mod stack;     pub use self::stack::*;
 //mod reduce;    pub use self::reduce::*;
 mod thunk;     pub use self::thunk::*;
diff --git a/output/src/ops/memo.rs b/output/src/ops/memo.rs
new file mode 100644
index 0000000..07e2d93
--- /dev/null
+++ b/output/src/ops/memo.rs
@@ -0,0 +1,30 @@
+use crate::*;
+use std::sync::{Arc, RwLock};
+
+#[derive(Debug, Default)] pub struct Memo<T, U> {
+    pub value: T,
+    pub view: Arc<RwLock<U>>
+}
+
+impl<T: PartialEq, U> Memo<T, U> {
+    pub fn new (value: T, view: U) -> Self {
+        Self { value, view: Arc::new(view.into()) }
+    }
+    pub fn update <R> (
+        &mut self,
+        newval: T,
+        render: impl Fn(&mut U, &T, &T)->R
+    ) -> Option<R> {
+        if newval != self.value {
+            let result = render(&mut*self.view.write().unwrap(), &newval, &self.value);
+            self.value = newval;
+            return Some(result);
+        }
+        None
+    }
+}
+
+/// Clear a pre-allocated buffer, then write into it.
+#[macro_export] macro_rules! rewrite {
+    ($buf:ident, $($rest:tt)*) => { |$buf,_,_|{ $buf.clear(); write!($buf, $($rest)*) } }
+}