diff --git a/implementations/rust/src/lib.rs b/implementations/rust/src/lib.rs
index acc9c43..210248c 100644
--- a/implementations/rust/src/lib.rs
+++ b/implementations/rust/src/lib.rs
@@ -2,12 +2,64 @@ pub mod value;
 pub mod symbol;
 
 #[cfg(test)]
-type Dom = ();
+mod dom {
+    use super::value::{Value, PlainValue, NestedValue, Domain, Encoder, Codec};
+
+    #[derive(Debug, Hash, Clone, Ord, PartialEq, Eq, PartialOrd)]
+    pub enum Dom {
+        One,
+        Two,
+    }
+
+    impl Domain for Dom {
+        fn encode<'a, 'b, W: std::io::Write, N: NestedValue<Self>>(
+            &self, enc: &mut Encoder<'a, 'b, W, N, Self>) ->
+            super::value::encoder::Result
+        {
+            match self {
+                Dom::One => enc.write_all(&[255, 255, 255, 255]),
+                Dom::Two => enc.write(&self.as_preserves()?)
+            }
+        }
+
+        fn as_preserves<N: NestedValue<Self>>(&self) -> Result<N, std::io::Error> {
+            Ok(Value::symbol(&format!("Dom::{:?}", self)).wrap())
+        }
+    }
+
+    #[test] fn test_one() {
+        let v: PlainValue<_> = Value::from(vec![Value::from(1).wrap(),
+                                                Value::Domain(Dom::One).wrap(),
+                                                Value::from(2).wrap()])
+            .wrap();
+        assert_eq!(Codec::without_placeholders().encode_bytes(&v).unwrap(),
+                   [147, 49, 255, 255, 255, 255, 50]);
+    }
+
+    #[test] fn test_two() {
+        let v: PlainValue<_> = Value::from(vec![Value::from(1).wrap(),
+                                                Value::Domain(Dom::Two).wrap(),
+                                                Value::from(2).wrap()])
+            .wrap();
+        assert_eq!(Codec::without_placeholders().encode_bytes(&v).unwrap(),
+                   [147, 49, 120, 68, 111, 109, 58, 58, 84, 119, 111, 50]);
+    }
+
+    #[test] fn test_unit() {
+        let v: PlainValue<_> = Value::from(vec![Value::from(1).wrap(),
+                                                Value::Domain(()).wrap(),
+                                                Value::from(2).wrap()])
+            .wrap();
+        let e = Codec::without_placeholders().encode_bytes(&v).err().unwrap();
+        assert_eq!(e.kind(), std::io::ErrorKind::InvalidData);
+        assert_eq!(e.to_string(), "Cannot Preserves-encode domain-specific value ()");
+    }
+}
 
 #[cfg(test)]
 mod ieee754_section_5_10_total_order_tests {
     use std::cmp::Ordering::{Less, Equal, Greater};
-    use super::Dom;
+    use super::dom::Dom;
 
     use crate::value::{Value, PlainValue};
     fn f(val: f32) -> Value<PlainValue<Dom>, Dom> { Value::from(val) }
@@ -103,7 +155,7 @@ mod ieee754_section_5_10_total_order_tests {
 mod value_tests {
     use crate::value::{Value, PlainValue};
     use num::bigint::BigInt;
-    use super::Dom;
+    use super::dom::Dom;
 
     type VV = Value<PlainValue<Dom>, Dom>;
 
@@ -184,7 +236,7 @@ mod value_tests {
 mod decoder_tests {
     use crate::value::Decoder;
     use crate::value::{Value, PlainValue, NestedValue};
-    use super::Dom;
+    use super::dom::Dom;
 
     #[test] fn read_123() {
         let mut buf = &b"abc"[..];
@@ -239,7 +291,7 @@ mod samples_tests {
     use crate::value::DecodePlaceholderMap;
     use crate::value::to_value;
     use crate::value::from_value;
-    use super::Dom;
+    use super::dom::Dom;
 
     #[derive(Debug, serde::Serialize, serde::Deserialize)]
     struct ExpectedPlaceholderMapping(DecodePlaceholderMap<PlainValue<Dom>, Dom>);
diff --git a/implementations/rust/src/value/encoder.rs b/implementations/rust/src/value/encoder.rs
index bb12aae..55b102e 100644
--- a/implementations/rust/src/value/encoder.rs
+++ b/implementations/rust/src/value/encoder.rs
@@ -10,8 +10,8 @@ pub type Result = std::result::Result<(), Error>;
 pub type EncodePlaceholderMap<N, D> = Map<Value<N, D>, usize>;
 
 pub struct Encoder<'a, 'b, W: Write, N: NestedValue<D>, D: Domain> {
-    write: &'a mut W,
-    placeholders: Option<&'b EncodePlaceholderMap<N, D>>,
+    pub write: &'a mut W,
+    pub placeholders: Option<&'b EncodePlaceholderMap<N, D>>,
 }
 
 impl<'a, 'b, W: Write, N: NestedValue<D>, D: Domain> Encoder<'a, 'b, W, N, D> {
@@ -117,10 +117,7 @@ impl<'a, 'b, W: Write, N: NestedValue<D>, D: Domain> Encoder<'a, 'b, W, N, D> {
                     for (k, v) in vs { self.write(k)?; self.write(v)?; }
                     Ok(())
                 }
-                Value::Domain(ref d) =>
-                    Err(Error::new(std::io::ErrorKind::InvalidData,
-                                   format!("Cannot Preserves-encode domain-specific value {:?}",
-                                           d))),
+                Value::Domain(ref d) => d.encode(self),
             }
         }
     }
diff --git a/implementations/rust/src/value/value.rs b/implementations/rust/src/value/value.rs
index d100c4c..5b6731e 100644
--- a/implementations/rust/src/value/value.rs
+++ b/implementations/rust/src/value/value.rs
@@ -11,7 +11,20 @@ use num::traits::cast::ToPrimitive;
 pub use std::collections::BTreeSet as Set;
 pub use std::collections::BTreeMap as Map;
 
-pub trait Domain: Sized + Debug + Clone + Eq + Hash + Ord {}
+pub trait Domain: Sized + Debug + Clone + Eq + Hash + Ord {
+    fn encode<'a, 'b, W: std::io::Write, N: NestedValue<Self>>(
+        &self, enc: &mut super::encoder::Encoder<'a, 'b, W, N, Self>) ->
+        super::encoder::Result
+    {
+        enc.write(&self.as_preserves()?)
+    }
+
+    fn as_preserves<N: NestedValue<Self>>(&self) -> Result<N, std::io::Error> {
+        Err(std::io::Error::new(std::io::ErrorKind::InvalidData,
+                                format!("Cannot Preserves-encode domain-specific value {:?}",
+                                        self)))
+    }
+}
 
 pub trait NestedValue<D: Domain>: Sized + Debug + Clone + Eq + Hash + Ord {
     type BoxType: Sized + Debug + Clone + Eq + Hash + Ord;