diff --git a/implementations/rust/Cargo.toml b/implementations/rust/Cargo.toml
index fdc97da..f1c13ae 100644
--- a/implementations/rust/Cargo.toml
+++ b/implementations/rust/Cargo.toml
@@ -1,5 +1,6 @@
 [workspace]
 members = [
+  "oo",
   "preserves",
   "preserves-path",
   "preserves-schema",
diff --git a/implementations/rust/oo/Cargo.toml b/implementations/rust/oo/Cargo.toml
new file mode 100644
index 0000000..0b4dad1
--- /dev/null
+++ b/implementations/rust/oo/Cargo.toml
@@ -0,0 +1,17 @@
+[package]
+name = "oo"
+version = "0.0.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+base64 = "0.13"
+dtoa = "0.4"
+num = "0.4"
+regex = "1.5"
+serde = { version = "1.0", features = ["derive"] }
+serde_bytes = "0.11"
+
+[package.metadata.workspaces]
+independent = true
diff --git a/implementations/rust/oo/src/float.rs b/implementations/rust/oo/src/float.rs
new file mode 100644
index 0000000..ff1f9cf
--- /dev/null
+++ b/implementations/rust/oo/src/float.rs
@@ -0,0 +1,94 @@
+use std::cmp::Ordering;
+use std::hash::{Hash, Hasher};
+
+/// Single-precision IEEE 754 Value
+#[derive(Clone, Copy, Debug)]
+pub struct Float(pub f32);
+
+/// Double-precision IEEE 754 Value
+#[derive(Clone, Copy, Debug)]
+pub struct Double(pub f64);
+
+impl From<f32> for Float {
+    fn from(v: f32) -> Self {
+        Float(v)
+    }
+}
+
+impl From<Float> for f32 {
+    fn from(v: Float) -> Self {
+        v.0
+    }
+}
+
+impl Hash for Float {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.0.to_bits().hash(state);
+    }
+}
+
+impl PartialEq for Float {
+    fn eq(&self, other: &Self) -> bool {
+        self.0.to_bits() == other.0.to_bits()
+    }
+}
+
+impl Ord for Float {
+    fn cmp(&self, other: &Self) -> Ordering {
+        let mut a: u32 = self.0.to_bits();
+        let mut b: u32 = other.0.to_bits();
+        if a & 0x8000_0000 != 0 { a ^= 0x7fff_ffff; }
+        if b & 0x8000_0000 != 0 { b ^= 0x7fff_ffff; }
+        (a as i32).cmp(&(b as i32))
+    }
+}
+
+impl PartialOrd for Float {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Eq for Float {}
+
+impl From<f64> for Double {
+    fn from(v: f64) -> Self {
+        Double(v)
+    }
+}
+
+impl From<Double> for f64 {
+    fn from(v: Double) -> Self {
+        v.0
+    }
+}
+
+impl Hash for Double {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.0.to_bits().hash(state);
+    }
+}
+
+impl PartialEq for Double {
+    fn eq(&self, other: &Self) -> bool {
+        self.0.to_bits() == other.0.to_bits()
+    }
+}
+
+impl Ord for Double {
+    fn cmp(&self, other: &Self) -> Ordering {
+        let mut a: u64 = self.0.to_bits();
+        let mut b: u64 = other.0.to_bits();
+        if a & 0x8000_0000_0000_0000 != 0 { a ^= 0x7fff_ffff_ffff_ffff; }
+        if b & 0x8000_0000_0000_0000 != 0 { b ^= 0x7fff_ffff_ffff_ffff; }
+        (a as i64).cmp(&(b as i64))
+    }
+}
+
+impl PartialOrd for Double {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Eq for Double {}
diff --git a/implementations/rust/oo/src/lib.rs b/implementations/rust/oo/src/lib.rs
new file mode 100644
index 0000000..b724062
--- /dev/null
+++ b/implementations/rust/oo/src/lib.rs
@@ -0,0 +1,430 @@
+use std::borrow::Cow;
+use std::cmp::Ordering;
+use std::fmt::Debug;
+use std::hash::{Hash, Hasher};
+use std::vec::Vec;
+
+pub use std::collections::BTreeSet as Set;
+pub use std::collections::BTreeMap as Map;
+
+pub mod float;
+pub mod signed_integer;
+
+pub use float::Float;
+pub use float::Double;
+pub use signed_integer::SignedInteger;
+
+/// The kinds of `Value` from the specification.
+#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub enum ValueClass {
+    Atomic(AtomClass),
+    Compound(CompoundClass),
+    Embedded,
+}
+
+/// The kinds of `Atom` from the specification.
+#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub enum AtomClass {
+    Boolean,
+    Float,
+    Double,
+    SignedInteger,
+    String,
+    ByteString,
+    Symbol,
+}
+
+/// The kinds of `Compound` from the specification.
+#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub enum CompoundClass {
+    Record,
+    Sequence,
+    Set,
+    Dictionary,
+}
+
+pub trait Domain: Debug + Eq + Hash + Ord + Clone {}
+
+#[derive(Debug)]
+pub struct OutOfRange;
+
+pub trait Value<D: Domain>: Debug {
+    fn value_class(&self) -> ValueClass;
+
+    fn as_boolean(&self) -> Option<bool> { None }
+    fn as_float(&self) -> Option<f32> { None }
+    fn as_double(&self) -> Option<f64> { None }
+
+    fn is_signed_integer(&self) -> bool { false }
+    fn as_signed_integer(&self) -> Option<Cow<'_, SignedInteger>> { None }
+
+    fn as_string(&self) -> Option<Cow<'_, str>> { None }
+    fn as_bytestring(&self) -> Option<Cow<'_, [u8]>> { None }
+    fn as_symbol(&self) -> Option<Cow<'_, str>> { None }
+
+    fn is_record(&self) -> bool { false }
+    fn label(&self) -> Option<&dyn Value<D>> { None }
+
+    fn is_sequence(&self) -> bool { false }
+    fn len(&self) -> Option<usize> { None }
+    fn index(&self, _i: usize) -> &dyn Value<D> { panic!("Not a sequence") }
+    fn iter(&self) -> Option<Box<dyn Iterator<Item = &dyn Value<D>> + '_>> { None }
+
+    fn is_set(&self) -> bool { false }
+    fn has(&self, _v: &dyn Value<D>) -> bool { false }
+
+    fn is_dictionary(&self) -> bool { false }
+    fn get<'v>(&'v self, _k: &'v dyn Value<D>) -> Option<&'v dyn Value<D>> { None }
+    fn entries(&self) -> Option<Box<dyn Iterator<Item = (&dyn Value<D>, &dyn Value<D>)> + '_>> { None }
+
+    fn is_embedded(&self) -> bool { false }
+    fn embedded(&self) -> Option<Cow<'_, D>> { None }
+
+    fn annotations(&self) -> Option<&[Box<dyn Value<D>>]> { None }
+}
+
+pub fn value<D: Domain, V: Value<D>>(v: &V) -> &dyn Value<D> {
+    v
+}
+
+impl<'a, D: Domain> Value<D> for &'a dyn Value<D> {
+    fn value_class(&self) -> ValueClass { (*self).value_class() }
+    fn as_boolean(&self) -> Option<bool> { (*self).as_boolean() }
+    fn as_float(&self) -> Option<f32> { (*self).as_float() }
+    fn as_double(&self) -> Option<f64> { (*self).as_double() }
+    fn is_signed_integer(&self) -> bool { (*self).is_signed_integer() }
+    fn as_signed_integer(&self) -> Option<Cow<'_, SignedInteger>> { (*self).as_signed_integer() }
+    fn as_string(&self) -> Option<Cow<'_, str>> { (*self).as_string() }
+    fn as_bytestring(&self) -> Option<Cow<'_, [u8]>> { (*self).as_bytestring() }
+    fn as_symbol(&self) -> Option<Cow<'_, str>> { (*self).as_symbol() }
+    fn is_record(&self) -> bool { (*self).is_record() }
+    fn label(&self) -> Option<&dyn Value<D>> { (*self).label() }
+    fn is_sequence(&self) -> bool { (*self).is_sequence() }
+    fn len(&self) -> Option<usize> { (*self).len() }
+    fn index(&self, i: usize) -> &dyn Value<D> { (*self).index(i) }
+    fn is_set(&self) -> bool { (*self).is_set() }
+    fn has(&self, v: &dyn Value<D>) -> bool { (*self).has(v) }
+    fn is_dictionary(&self) -> bool { (*self).is_dictionary() }
+    fn get<'v>(&'v self, k: &'v dyn Value<D>) -> Option<&'v dyn Value<D>> { (*self).get(k) }
+    fn entries(&self) -> Option<Box<dyn Iterator<Item = (&dyn Value<D>, &dyn Value<D>)> + '_>> { (*self).entries() }
+    fn is_embedded(&self) -> bool { (*self).is_embedded() }
+    fn embedded(&self) -> Option<Cow<'_, D>> { (*self).embedded() }
+    fn annotations(&self) -> Option<&[Box<dyn Value<D>>]> { (*self).annotations() }
+}
+
+impl<D: Domain> Value<D> for Box<dyn Value<D>> {
+    fn value_class(&self) -> ValueClass { self.as_ref().value_class() }
+    fn as_boolean(&self) -> Option<bool> { self.as_ref().as_boolean() }
+    fn as_float(&self) -> Option<f32> { self.as_ref().as_float() }
+    fn as_double(&self) -> Option<f64> { self.as_ref().as_double() }
+    fn is_signed_integer(&self) -> bool { self.as_ref().is_signed_integer() }
+    fn as_signed_integer(&self) -> Option<Cow<'_, SignedInteger>> { self.as_ref().as_signed_integer() }
+    fn as_string(&self) -> Option<Cow<'_, str>> { self.as_ref().as_string() }
+    fn as_bytestring(&self) -> Option<Cow<'_, [u8]>> { self.as_ref().as_bytestring() }
+    fn as_symbol(&self) -> Option<Cow<'_, str>> { self.as_ref().as_symbol() }
+    fn is_record(&self) -> bool { self.as_ref().is_record() }
+    fn label(&self) -> Option<&dyn Value<D>> { self.as_ref().label() }
+    fn is_sequence(&self) -> bool { self.as_ref().is_sequence() }
+    fn len(&self) -> Option<usize> { self.as_ref().len() }
+    fn index(&self, i: usize) -> &dyn Value<D> { self.as_ref().index(i) }
+    fn is_set(&self) -> bool { self.as_ref().is_set() }
+    fn has(&self, v: &dyn Value<D>) -> bool { self.as_ref().has(v) }
+    fn is_dictionary(&self) -> bool { self.as_ref().is_dictionary() }
+    fn get<'v>(&'v self, k: &'v dyn Value<D>) -> Option<&'v dyn Value<D>> { self.as_ref().get(k) }
+    fn entries(&self) -> Option<Box<dyn Iterator<Item = (&dyn Value<D>, &dyn Value<D>)> + '_>> { self.as_ref().entries() }
+    fn is_embedded(&self) -> bool { self.as_ref().is_embedded() }
+    fn embedded(&self) -> Option<Cow<'_, D>> { self.as_ref().embedded() }
+    fn annotations(&self) -> Option<&[Box<dyn Value<D>>]> { self.as_ref().annotations() }
+}
+
+impl<'a, D: Domain> Hash for dyn Value<D> + 'a {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        match self.value_class() {
+            ValueClass::Atomic(a) => match a {
+                AtomClass::Boolean => self.as_boolean().unwrap().hash(state),
+                AtomClass::Float => self.as_float().unwrap().to_bits().hash(state),
+                AtomClass::Double => self.as_double().unwrap().to_bits().hash(state),
+                AtomClass::SignedInteger => self.as_signed_integer().unwrap().hash(state),
+                AtomClass::String => self.as_string().unwrap().hash(state),
+                AtomClass::ByteString => self.as_bytestring().unwrap().hash(state),
+                AtomClass::Symbol => self.as_symbol().unwrap().hash(state),
+            }
+            ValueClass::Compound(c) => match c {
+                CompoundClass::Sequence |
+                CompoundClass::Set => {
+                    state.write_usize(self.len().unwrap());
+                    for v in self.iter().unwrap() { v.hash(state) }
+                }
+                CompoundClass::Record => {
+                    self.label().unwrap().hash(state);
+                    state.write_usize(self.len().unwrap());
+                    for v in self.iter().unwrap() { v.hash(state) }
+                }
+                CompoundClass::Dictionary => {
+                    state.write_usize(self.len().unwrap());
+                    for (k, v) in self.entries().unwrap() {
+                        k.hash(state);
+                        v.hash(state);
+                    }
+                }
+            }
+            ValueClass::Embedded => self.embedded().unwrap().hash(state),
+        }
+    }
+}
+
+impl<'a, D: Domain> PartialEq for dyn Value<D> + 'a {
+    fn eq(&self, other: &Self) -> bool {
+        let cls = self.value_class();
+        if cls != other.value_class() { return false; }
+        match cls {
+            ValueClass::Atomic(a) => match a {
+                AtomClass::Boolean =>
+                    self.as_boolean().unwrap() == other.as_boolean().unwrap(),
+                AtomClass::Float =>
+                    Float(self.as_float().unwrap()) == Float(other.as_float().unwrap()),
+                AtomClass::Double =>
+                    Double(self.as_double().unwrap()) == Double(other.as_double().unwrap()),
+                AtomClass::SignedInteger =>
+                    self.as_signed_integer().unwrap() == other.as_signed_integer().unwrap(),
+                AtomClass::String =>
+                    self.as_string().unwrap() == other.as_string().unwrap(),
+                AtomClass::ByteString =>
+                    self.as_bytestring().unwrap() == other.as_bytestring().unwrap(),
+                AtomClass::Symbol =>
+                    self.as_symbol().unwrap() == other.as_symbol().unwrap(),
+            }
+            ValueClass::Compound(c) => match c {
+                _ => todo!(),
+            }
+            ValueClass::Embedded => self.embedded().unwrap() == other.embedded().unwrap(),
+        }
+    }
+}
+
+impl<'a, D: Domain> Ord for dyn Value<D> + 'a {
+    fn cmp(&self, other: &Self) -> Ordering {
+        let cls = self.value_class();
+        match cls.cmp(&other.value_class()) {
+            Ordering::Less => Ordering::Less,
+            Ordering::Greater => Ordering::Greater,
+            Ordering::Equal => match cls {
+                ValueClass::Atomic(a) => match a {
+                    AtomClass::Boolean =>
+                        self.as_boolean().cmp(&other.as_boolean()),
+                    AtomClass::Float =>
+                        Float(self.as_float().unwrap()).cmp(&Float(other.as_float().unwrap())),
+                    AtomClass::Double =>
+                        Double(self.as_double().unwrap()).cmp(&Double(other.as_double().unwrap())),
+                    AtomClass::SignedInteger =>
+                        self.as_signed_integer().cmp(&other.as_signed_integer()),
+                    AtomClass::String =>
+                        self.as_string().cmp(&other.as_string()),
+                    AtomClass::ByteString =>
+                        self.as_bytestring().cmp(&other.as_bytestring()),
+                    AtomClass::Symbol =>
+                        self.as_symbol().cmp(&other.as_symbol()),
+                },
+                ValueClass::Compound(c) => match c {
+                    _ => todo!(),
+                },
+                ValueClass::Embedded =>
+                    self.embedded().cmp(&other.embedded()),
+            }
+        }
+    }
+}
+
+impl<'a, D: Domain> Eq for dyn Value<D> + 'a {}
+
+impl<'a, D: Domain> PartialOrd for dyn Value<D> + 'a {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub enum NoValue {}
+
+impl Domain for NoValue {}
+
+impl<D: Domain> Value<D> for NoValue {
+    fn value_class(&self) -> ValueClass { unreachable!() }
+}
+
+impl<D: Domain> Value<D> for bool {
+    fn value_class(&self) -> ValueClass { ValueClass::Atomic(AtomClass::Boolean) }
+    fn as_boolean(&self) -> Option<bool> { Some(*self) }
+}
+
+impl<D: Domain> Value<D> for u64 {
+    fn value_class(&self) -> ValueClass { ValueClass::Atomic(AtomClass::SignedInteger) }
+    fn as_signed_integer(&self) -> Option<Cow<'_, SignedInteger>> {
+        Some(Cow::Owned((*self).into()))
+    }
+}
+
+impl<D: Domain> Value<D> for Float {
+    fn value_class(&self) -> ValueClass { ValueClass::Atomic(AtomClass::Float) }
+    fn as_float(&self) -> Option<f32> { Some(self.0) }
+    fn as_double(&self) -> Option<f64> { Some(self.0 as f64) }
+}
+
+impl<'a, D: Domain> Value<D> for &'a str {
+    fn value_class(&self) -> ValueClass { ValueClass::Atomic(AtomClass::String) }
+    fn as_string(&self) -> Option<Cow<'_, str>> { Some(Cow::Borrowed(self)) }
+}
+
+impl<D: Domain> Value<D> for String {
+    fn value_class(&self) -> ValueClass { ValueClass::Atomic(AtomClass::String) }
+    fn as_string(&self) -> Option<Cow<'_, str>> { Some(Cow::Borrowed(self)) }
+}
+
+impl<D: Domain, V: Value<D>> Value<D> for Vec<V> {
+    fn value_class(&self) -> ValueClass { ValueClass::Compound(CompoundClass::Sequence) }
+    fn is_sequence(&self) -> bool { true }
+    fn len(&self) -> Option<usize> { Some(self.len()) }
+    fn index(&self, i: usize) -> &dyn Value<D> { &self[i] }
+    fn iter(&self) -> Option<Box<dyn Iterator<Item = &dyn Value<D>> + '_>> {
+        Some(Box::new(self[..].iter().map(value)))
+    }
+}
+
+impl<D: Domain, V: Value<D>> Value<D> for [V] {
+    fn value_class(&self) -> ValueClass { ValueClass::Compound(CompoundClass::Sequence) }
+    fn is_sequence(&self) -> bool { true }
+    fn len(&self) -> Option<usize> { Some(self.len()) }
+    fn index(&self, i: usize) -> &dyn Value<D> { &self[i] }
+    fn iter(&self) -> Option<Box<dyn Iterator<Item = &dyn Value<D>> + '_>> {
+        Some(Box::new(self[..].iter().map(value)))
+    }
+}
+
+impl<'v, D: Domain, V: Value<D>> Value<D> for &'v [V] {
+    fn value_class(&self) -> ValueClass { ValueClass::Compound(CompoundClass::Sequence) }
+    fn is_sequence(&self) -> bool { true }
+    fn len(&self) -> Option<usize> { Some((self as &[V]).len()) }
+    fn index(&self, i: usize) -> &dyn Value<D> { &self[i] }
+    fn iter(&self) -> Option<Box<dyn Iterator<Item = &dyn Value<D>> + '_>> {
+        Some(Box::new(self[..].iter().map(value)))
+    }
+}
+
+impl<D: Domain> Value<D> for Set<Box<dyn Value<D>>> {
+    fn value_class(&self) -> ValueClass { ValueClass::Compound(CompoundClass::Set) }
+    fn is_set(&self) -> bool { true }
+    fn len(&self) -> Option<usize> { Some(self.len()) }
+    fn has(&self, v: &dyn Value<D>) -> bool { self.contains(v) }
+    fn iter(&self) -> Option<Box<dyn Iterator<Item = &dyn Value<D>> + '_>> {
+        Some(Box::new(self.iter().map(value)))
+    }
+}
+
+impl<D: Domain, V: Value<D>> Value<D> for Map<Box<dyn Value<D>>, V> {
+    fn value_class(&self) -> ValueClass { ValueClass::Compound(CompoundClass::Dictionary) }
+    fn is_dictionary(&self) -> bool { true }
+    fn len(&self) -> Option<usize> { Some(self.len()) }
+    fn has(&self, v: &dyn Value<D>) -> bool { self.contains_key(v) }
+    fn get<'v>(&'v self, k: &'v dyn Value<D>) -> Option<&'v dyn Value<D>> {
+        match Map::get(self, k) {
+            Some(v) => Some(v),
+            None => None,
+        }
+    }
+    fn entries(&self) -> Option<Box<dyn Iterator<Item = (&dyn Value<D>, &dyn Value<D>)> + '_>> {
+        Some(Box::new(self.iter().map(|(k,v)| (value(k), value(v)))))
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub struct Record<V>(Vec<V> /* at least one element, for the label */);
+
+impl<D: Domain, V: Value<D>> Value<D> for Record<V> {
+    fn value_class(&self) -> ValueClass { ValueClass::Compound(CompoundClass::Record) }
+    fn is_record(&self) -> bool { true }
+    fn label(&self) -> Option<&dyn Value<D>> { Some(&self.0[0]) }
+    fn len(&self) -> Option<usize> { Some(self.0.len() - 1) }
+    fn index(&self, i: usize) -> &dyn Value<D> { &self.0[i + 1] }
+}
+
+#[derive(Debug)]
+pub struct Annotations<D: Domain, V: Value<D>>(V, Vec<Box<dyn Value<D>>>);
+
+impl<D: Domain, V: Value<D>> Annotations<D, V> {
+    pub fn value(&self) -> &dyn Value<D> {
+        &self.0
+    }
+}
+
+impl<D: Domain, V: Value<D>> Value<D> for Annotations<D, V> {
+    fn value_class(&self) -> ValueClass { self.value().value_class() }
+    fn as_boolean(&self) -> Option<bool> { self.value().as_boolean() }
+    fn as_float(&self) -> Option<f32> { self.value().as_float() }
+    fn as_double(&self) -> Option<f64> { self.value().as_double() }
+    fn is_signed_integer(&self) -> bool { self.value().is_signed_integer() }
+    fn as_signed_integer(&self) -> Option<Cow<'_, SignedInteger>> { self.value().as_signed_integer() }
+    fn as_string(&self) -> Option<Cow<'_, str>> { self.value().as_string() }
+    fn as_bytestring(&self) -> Option<Cow<'_, [u8]>> { self.value().as_bytestring() }
+    fn as_symbol(&self) -> Option<Cow<'_, str>> { self.value().as_symbol() }
+    fn is_record(&self) -> bool { self.value().is_record() }
+    fn label(&self) -> Option<&dyn Value<D>> { self.value().label() }
+    fn is_sequence(&self) -> bool { self.value().is_sequence() }
+    fn len(&self) -> Option<usize> { self.value().len() }
+    fn index(&self, i: usize) -> &dyn Value<D> { self.value().index(i) }
+    fn is_set(&self) -> bool { self.value().is_set() }
+    fn has(&self, v: &dyn Value<D>) -> bool { self.value().has(v) }
+    fn is_dictionary(&self) -> bool { self.value().is_dictionary() }
+    fn get<'v>(&'v self, k: &'v dyn Value<D>) -> Option<&'v dyn Value<D>> { self.value().get(k) }
+    fn entries(&self) -> Option<Box<dyn Iterator<Item = (&dyn Value<D>, &dyn Value<D>)> + '_>> { self.value().entries() }
+    fn is_embedded(&self) -> bool { self.value().is_embedded() }
+    fn embedded(&self) -> Option<Cow<'_, D>> { self.value().embedded() }
+    fn annotations(&self) -> Option<&[Box<dyn Value<D>>]> { Some(&self.1) }
+}
+
+impl<D: Domain, V: Value<D>> PartialEq for Annotations<D, V> {
+    fn eq(&self, other: &Self) -> bool {
+        self.value().eq(&other.value())
+    }
+}
+
+impl<D: Domain, V: Value<D>> Eq for Annotations<D, V> {}
+
+impl<D: Domain, V: Value<D>> Hash for Annotations<D, V> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.value().hash(state);
+    }
+}
+
+impl<D: Domain, V: Value<D>> PartialOrd for Annotations<D, V> {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl<D: Domain, V: Value<D>> Ord for Annotations<D, V> {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.value().cmp(&other.value())
+    }
+}
+
+#[cfg(test)]
+mod demo {
+    use crate::*;
+
+    #[test] fn a() {
+        let mut v: Map<Box<dyn Value<NoValue>>, String> = Map::new();
+        v.insert(Box::new("abc"), "def".to_owned());
+        v.insert(Box::new(123), "xyz".to_owned());
+        v.insert(Box::new(vec![1, 2, 3]), "www".to_owned());
+        let w: &dyn Value<NoValue> = &v;
+        println!("GETw abc {:?}", w.get(&"abc"));
+        println!("GETw 123 {:?}", w.get(&123));
+        println!("GETw qqq {:?}", w.get(&"qqq"));
+        println!("GETv abc {:?}", v.get(value(&"abc")));
+        println!("GETv 123 {:?}", v.get(value(&123)));
+        println!("GETv qqq {:?}", v.get(value(&"qqq")));
+        for (kk, vv) in w.entries().unwrap() {
+            println!("{:?} {:?} ==> {:?} {:?}", kk.value_class(), kk, vv.value_class(), vv);
+        }
+    }
+}
diff --git a/implementations/rust/oo/src/signed_integer.rs b/implementations/rust/oo/src/signed_integer.rs
new file mode 100644
index 0000000..9c753e5
--- /dev/null
+++ b/implementations/rust/oo/src/signed_integer.rs
@@ -0,0 +1,229 @@
+use num::bigint::BigInt;
+use num::traits::cast::ToPrimitive;
+use num::traits::sign::Signed;
+use std::borrow::Cow;
+use std::cmp::{Ord, Ordering, PartialOrd};
+use std::convert::TryFrom;
+use std::convert::TryInto;
+use std::fmt;
+
+// Invariant: if I128 can be used, it will be; otherwise, if U128 can
+// be used, it will be; otherwise, Big will be used.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub enum SignedIntegerRepr {
+    I128(i128),
+    U128(u128),
+    Big(Box<BigInt>),
+}
+
+#[derive(Clone, PartialEq, Eq, Hash)]
+pub struct SignedInteger(SignedIntegerRepr);
+
+impl fmt::Debug for SignedInteger {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        write!(f, "{}n", self)
+    }
+}
+
+impl fmt::Display for SignedInteger {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        match self.repr() {
+            SignedIntegerRepr::I128(i) => i.fmt(f),
+            SignedIntegerRepr::U128(u) => u.fmt(f),
+            SignedIntegerRepr::Big(n) => n.fmt(f),
+        }
+    }
+}
+
+impl Ord for SignedInteger {
+    fn cmp(&self, other: &Self) -> Ordering {
+        match self.repr() {
+            SignedIntegerRepr::I128(i1) => match other.repr() {
+                SignedIntegerRepr::I128(i2) => i1.cmp(i2),
+                SignedIntegerRepr::U128(_) => if *i1 < 0 { Ordering::Less } else { Ordering::Greater },
+                SignedIntegerRepr::Big(n) => if n.is_negative() { Ordering::Less } else { Ordering::Greater },
+            },
+            SignedIntegerRepr::U128(u1) => match other.repr() {
+                SignedIntegerRepr::I128(_) => Ordering::Greater,
+                SignedIntegerRepr::U128(u2) => u1.cmp(u2),
+                SignedIntegerRepr::Big(n) => if n.is_positive() { Ordering::Less } else { Ordering::Greater },
+            },
+            SignedIntegerRepr::Big(n1) => match other.repr() {
+                SignedIntegerRepr::I128(_) |
+                SignedIntegerRepr::U128(_) => if n1.is_negative() { Ordering::Less } else { Ordering::Greater },
+                SignedIntegerRepr::Big(n2) => n1.cmp(n2),
+            },
+        }
+    }
+}
+
+impl PartialOrd for SignedInteger {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl SignedInteger {
+    pub fn repr(&self) -> &SignedIntegerRepr {
+        &self.0
+    }
+
+    pub fn is_i(&self) -> bool {
+        matches!(self.0, SignedIntegerRepr::I128(_))
+    }
+
+    pub fn is_u(&self) -> bool {
+        matches!(self.0, SignedIntegerRepr::U128(_))
+    }
+
+    pub fn is_big(&self) -> bool {
+        matches!(self.0, SignedIntegerRepr::Big(_))
+    }
+}
+
+//--
+
+macro_rules! map_integral_type_to_signed_integer {
+    ($iN:ident, $uN:ident) => {
+        impl From<$iN> for SignedInteger {
+            fn from(v: $iN) -> Self {
+                SignedInteger(SignedIntegerRepr::I128(v.into()))
+            }
+        }
+
+        impl From<$uN> for SignedInteger {
+            fn from(v: $uN) -> Self {
+                SignedInteger(SignedIntegerRepr::I128(v.into()))
+            }
+        }
+
+        impl TryFrom<&SignedInteger> for $iN {
+            type Error = ();
+            fn try_from(v: &SignedInteger) -> Result<Self, Self::Error> {
+                i128::try_from(v)?.try_into().map_err(|_| ())
+            }
+        }
+
+        impl TryFrom<&SignedInteger> for $uN {
+            type Error = ();
+            fn try_from(v: &SignedInteger) -> Result<Self, Self::Error> {
+                u128::try_from(v)?.try_into().map_err(|_| ())
+            }
+        }
+    }
+}
+
+map_integral_type_to_signed_integer!(i8, u8);
+map_integral_type_to_signed_integer!(i16, u16);
+map_integral_type_to_signed_integer!(i32, u32);
+map_integral_type_to_signed_integer!(i64, u64);
+
+//--
+
+impl From<i128> for SignedInteger {
+    fn from(v: i128) -> Self {
+        SignedInteger(SignedIntegerRepr::I128(v))
+    }
+}
+
+impl From<u128> for SignedInteger {
+    fn from(v: u128) -> Self {
+        if let Ok(w) = v.try_into() {
+            SignedInteger(SignedIntegerRepr::I128(w))
+        } else {
+            SignedInteger(SignedIntegerRepr::U128(v))
+        }
+    }
+}
+
+impl TryFrom<&SignedInteger> for i128 {
+    type Error = ();
+    fn try_from(v: &SignedInteger) -> Result<Self, Self::Error> {
+        match v.repr() {
+            SignedIntegerRepr::I128(i) => Ok(*i),
+            SignedIntegerRepr::U128(_) => Err(()),
+            SignedIntegerRepr::Big(_) => Err(()),
+        }
+    }
+}
+
+impl TryFrom<&SignedInteger> for u128 {
+    type Error = ();
+    fn try_from(v: &SignedInteger) -> Result<Self, Self::Error> {
+        match v.repr() {
+            SignedIntegerRepr::I128(i) => i.to_u128().ok_or(()),
+            SignedIntegerRepr::U128(u) => Ok(*u),
+            SignedIntegerRepr::Big(_) => Err(()),
+        }
+    }
+}
+
+//--
+
+impl From<BigInt> for SignedInteger {
+    fn from(v: BigInt) -> Self {
+        Self::from(Cow::Owned(v))
+    }
+}
+
+impl<'a> From<&'a SignedInteger> for BigInt {
+    fn from(v: &'a SignedInteger) -> Self {
+        match v.repr() {
+            SignedIntegerRepr::I128(i) => BigInt::from(*i),
+            SignedIntegerRepr::U128(u) => BigInt::from(*u),
+            SignedIntegerRepr::Big(n) => *n.clone(),
+        }
+    }
+}
+
+//--
+
+impl<'a> From<Cow<'a, BigInt>> for SignedInteger {
+    fn from(v: Cow<'a, BigInt>) -> Self {
+        if let Some(w) = v.to_i128() {
+            SignedInteger(SignedIntegerRepr::I128(w))
+        } else if let Some(w) = v.to_u128() {
+            SignedInteger(SignedIntegerRepr::U128(w))
+        } else {
+            SignedInteger(SignedIntegerRepr::Big(Box::new(v.into_owned())))
+        }
+    }
+}
+
+impl<'a> From<&'a SignedInteger> for Cow<'a, BigInt> {
+    fn from(v: &'a SignedInteger) -> Self {
+        match v.repr() {
+            SignedIntegerRepr::I128(i) => Cow::Owned(BigInt::from(*i)),
+            SignedIntegerRepr::U128(u) => Cow::Owned(BigInt::from(*u)),
+            SignedIntegerRepr::Big(n) => Cow::Borrowed(n),
+        }
+    }
+}
+
+//--
+
+impl From<isize> for SignedInteger {
+    fn from(v: isize) -> Self {
+        SignedInteger(SignedIntegerRepr::I128(v as i128))
+    }
+}
+
+impl From<usize> for SignedInteger {
+    fn from(v: usize) -> Self {
+        SignedInteger(SignedIntegerRepr::U128(v as u128))
+    }
+}
+
+impl TryFrom<&SignedInteger> for isize {
+    type Error = ();
+    fn try_from(v: &SignedInteger) -> Result<Self, Self::Error> {
+        i128::try_from(v)?.try_into().map_err(|_| ())
+    }
+}
+
+impl TryFrom<&SignedInteger> for usize {
+    type Error = ();
+    fn try_from(v: &SignedInteger) -> Result<Self, Self::Error> {
+        u128::try_from(v)?.try_into().map_err(|_| ())
+    }
+}