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diff --git a/receme/src/catana.rs b/receme/src/catana.rs
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+//! This file defines behaviours of catamorphisms and anamorphisms.
+//!
+//! A catamorphism collapses a recursive structure into a flat value,
+//! whereas an anamorphism expands a value into a recursive structure.
+
+use crate::{algebra::Algebra, coalgebra::Coalgebra, functor::Functor};
+
+/// A type that implements Cata is capable of being collapsed into a single value.
+///
+/// The catamorphism consumes `self`.
+pub trait Cata<T, F: Functor<T>, A: Algebra<T, F>> {
+    /// A catamorphism takes a recursive structure and an algebra for
+    /// the recursive structure, and returns a single, flat, collapsed
+    /// value.
+    fn cata(self, alg: A) -> T;
+}
+
+/// A type that implements Ana is capable of being expanded from a
+/// flat value.
+///
+/// The anamorphism consumes `self`.
+pub trait Ana<T, F: Functor<T>, C: Coalgebra<T, F>> {
+    /// An anamorphism takes a single, flat, collapsed value and a
+    /// co-algebra for a recursive structure, and returns that
+    /// recursive structure.
+    fn ana(value: T, coalg: C) -> Self;
+}