diff --git a/rust/adagio/src/eval.rs b/rust/adagio/src/eval.rs
--- a/rust/adagio/src/eval.rs
+++ b/rust/adagio/src/eval.rs
@@ -0,0 +1,28 @@
+use std::marker::PhantomData;
+
+use void::Void;
+
+trait Input<I, T> {
+    fn get(&self, index: I) -> T where T: Clone;
+}
+
+trait Parameter<I, T> {
+    fn get(&self, index: I) -> T where T: Clone;
+}
+
+trait Constant<I, T> {
+    fn get(&self, index: I) -> T where T: Clone;
+}
+
+struct InputVec<T> (Vec<T>);
+struct ParamVec<T> (Vec<T>);
+struct UninhabitableConstant<T> (Void, PhantomData<T>);
+
+trait Eval<T> {
+    type InputIndex;
+    type Input: Input<Self::InputIndex, T>;
+    type ParameterIndex;
+    type Parameter: Parameter<Self::ParameterIndex, T>;
+    type ConstantIndex;
+    type Constant: Constant<Self::ConstantIndex, T>;
+}
diff --git a/rust/adagio/src/lib.rs b/rust/adagio/src/lib.rs
--- a/rust/adagio/src/lib.rs
+++ b/rust/adagio/src/lib.rs
@@ -1,9 +1,11 @@
 extern crate num;
+extern crate void;

 use std::cmp::Ordering;
 use std::collections::HashMap;
 use std::hash::Hash;

+mod eval;
 mod node_conversions;
 mod node_defs;
 mod node_ordering;
@@ -68,10 +70,7 @@
         if self.minus {
             env.negate_products(self.terms.iter().cloned())
         } else {
-            self.terms
-                .iter()
-                .map(|p| p.insert(env))
-                .collect()
+            self.terms.iter().map(|p| p.insert(env)).collect()
         }
     }
 }
@@ -141,9 +140,9 @@
         }
     }

-    /// Makes a canonicalized SystemVariable from the given components.
-    pub fn make_system_variable(&mut self, index: T::SystemVariable) -> Container<Primitive<T>> {
-        Primitive::SystemVariable(index).insert(self)
+    /// Makes a canonicalized Real from the given components.
+    pub fn make_real(&mut self, index: T::Real) -> Container<Primitive<T>> {
+        Primitive::Real(index).insert(self)
     }

     /// Makes a canonicalized Parameter from the given components.
@@ -355,15 +354,13 @@
                           -> Container<Sum<T>>
         where Self: Sized + Clone + Eq + Hash
     {
-        if !self.get_derivative_cache(env).contains_key(&(self.clone(), variable.clone())) {
+        if !self.get_derivative_cache(env)
+                .contains_key(&(self.clone(), variable.clone())) {
             let key = (self.fancy_clone(env), variable.clone());
             let result = self.derivative_impl(variable, env);
             self.get_derivative_cache(env).insert(key, result);
         }
-        match self.get_derivative_cache(env).get(&(self.clone(), variable.clone())) {
-            Some(derivative) => derivative.clone(),
-            None => unreachable!(),
-        }
+        self.get_derivative_cache(env)[&(self.clone(), variable.clone())].clone()
     }

     fn get_storage<'a>(&self, env: &'a mut Environment<T>) -> &'a mut SelfMap<Self>
@@ -387,10 +384,7 @@
             let value = key.clone();
             self.get_storage(env).insert(key, container(value));
         }
-        match self.get_storage(env).get(self) {
-            Some(node) => node.clone(),
-            None => unreachable!(),
-        }
+        self.get_storage(env)[self].clone()
     }
 }

diff --git a/rust/adagio/src/node_defs.rs b/rust/adagio/src/node_defs.rs
--- a/rust/adagio/src/node_defs.rs
+++ b/rust/adagio/src/node_defs.rs
@@ -18,7 +18,7 @@
     type Coefficient: Hash + Ord + Clone + Signed + Into<Self::Constant> + From<i32>;
     type Exponent: Hash + Clone + Integer + Into<Self::Constant> + From<i32>;
     type Input: Hash + Ord + Clone;
-    type SystemVariable: Hash + Ord + Clone;
+    type Real: Hash + Ord + Clone;
     type Parameter: Hash + Ord + Clone;
 }

@@ -27,14 +27,14 @@
 }

 pub struct Sum<T: NodeData> {
-    pre_hash: u64, //I would like these fields to be private if possible.
+    pre_hash: u64,
     pub minus: bool,
     pub constant: T::Constant,
     pub terms: Vec<Container<Product<T>>>,
 }

 pub struct Product<T: NodeData> {
-    pre_hash: u64, //I would like these fields to be private if possible.
+    pre_hash: u64,
     pub coefficient: T::Coefficient,
     pub powers: Vec<Container<Power<T>>>,
 }
@@ -46,7 +46,7 @@

 pub enum Primitive<T: NodeData> {
     Input(T::Input),
-    SystemVariable(T::SystemVariable),
+    Real(T::Real),
     Parameter(T::Parameter),
     Sigmoid(bool, Container<Sum<T>>),
 }
@@ -94,7 +94,7 @@
     fn eq(&self, other: &Primitive<T>) -> bool {
         match (self, other) {
             (&Primitive::Input(ref a), &Primitive::Input(ref b)) => a == b,
-            (&Primitive::SystemVariable(ref a), &Primitive::SystemVariable(ref b)) => a == b,
+            (&Primitive::Real(ref a), &Primitive::Real(ref b)) => a == b,
             (&Primitive::Parameter(ref a), &Primitive::Parameter(ref b)) => a == b,
             (&Primitive::Sigmoid(minus_a, ref a), &Primitive::Sigmoid(minus_b, ref b)) => {
                 minus_a == minus_b && a == b
@@ -135,8 +135,8 @@
                 "Input".hash(state);
                 a.hash(state)
             }
-            &Primitive::SystemVariable(ref a) => {
-                "SystemVariable".hash(state);
+            &Primitive::Real(ref a) => {
+                "Real".hash(state);
                 a.hash(state)
             }
             &Primitive::Parameter(ref a) => {
@@ -158,10 +158,7 @@
             pre_hash: self.pre_hash.clone(),
             minus: self.minus.clone(),
             constant: self.constant.clone(),
-            terms: self.terms
-                .iter()
-                .map(|p| p.insert(env))
-                .collect(),
+            terms: self.terms.iter().map(|p| p.insert(env)).collect(),
         }
     }
 }
@@ -171,10 +168,7 @@
         Product {
             pre_hash: self.pre_hash.clone(),
             coefficient: self.coefficient.clone(),
-            powers: self.powers
-                .iter()
-                .map(|p| p.insert(env))
-                .collect(),
+            powers: self.powers.iter().map(|p| p.insert(env)).collect(),
         }
     }
 }
@@ -231,7 +225,7 @@
     fn clone(&self) -> Primitive<T> {
         match self {
             &Primitive::Input(ref a) => Primitive::Input(a.clone()),
-            &Primitive::SystemVariable(ref a) => Primitive::SystemVariable(a.clone()),
+            &Primitive::Real(ref a) => Primitive::Real(a.clone()),
             &Primitive::Parameter(ref a) => Primitive::Parameter(a.clone()),
             &Primitive::Sigmoid(minus, ref a) => Primitive::Sigmoid(minus, a.clone()),
         }
diff --git a/rust/adagio/src/node_ordering.rs b/rust/adagio/src/node_ordering.rs
--- a/rust/adagio/src/node_ordering.rs
+++ b/rust/adagio/src/node_ordering.rs
@@ -5,7 +5,7 @@
 #[derive(Eq, Ord, PartialEq, PartialOrd)]
 enum PrimitiveType {
     Input,
-    SystemVariable,
+    Real,
     Parameter,
     Sigmoid,
 }
@@ -14,7 +14,7 @@
     fn simplify(&self) -> PrimitiveType {
         match self {
             &Primitive::Input(_) => PrimitiveType::Input,
-            &Primitive::SystemVariable(_) => PrimitiveType::SystemVariable,
+            &Primitive::Real(_) => PrimitiveType::Real,
             &Primitive::Parameter(_) => PrimitiveType::Parameter,
             &Primitive::Sigmoid(_, _) => PrimitiveType::Sigmoid,
         }
@@ -27,10 +27,7 @@
         match cmp_result {
             Ordering::Equal => {
                 for (self_term, other_term) in
-                    self.terms
-                        .clone()
-                        .into_iter()
-                        .zip(other.terms.clone()) {
+                    self.terms.clone().into_iter().zip(other.terms.clone()) {
                     let cmp_result = self_term.cmp(&other_term);
                     match cmp_result {
                         Ordering::Equal => (),
@@ -94,7 +91,7 @@
     fn cmp(&self, other: &Primitive<T>) -> Ordering {
         match (self, other) {
             (&Primitive::Input(ref a), &Primitive::Input(ref b)) => a.cmp(b),
-            (&Primitive::SystemVariable(ref a), &Primitive::SystemVariable(ref b)) => a.cmp(b),
+            (&Primitive::Real(ref a), &Primitive::Real(ref b)) => a.cmp(b),
             (&Primitive::Parameter(ref a), &Primitive::Parameter(ref b)) => a.cmp(b),
             (&Primitive::Sigmoid(minus_a, ref a), &Primitive::Sigmoid(minus_b, ref b)) => {
                 let cmp_result = a.cmp(b);