diff --git a/src/formula/mod.rs b/src/formula/mod.rs
index b0d4024..bd2281c 100644
--- a/src/formula/mod.rs
+++ b/src/formula/mod.rs
@@ -3,7 +3,7 @@ use crate::formula::node::Node;
 pub mod node;
 
 pub struct Formula {
-    root: node::Node,
+    root: node::Node<F>,
 }
 
 impl Formula {
diff --git a/src/formula/node/function.rs b/src/formula/node/function.rs
new file mode 100644
index 0000000..f8ba181
--- /dev/null
+++ b/src/formula/node/function.rs
@@ -0,0 +1,3 @@
+pub trait Function {
+    fn map(x: f64) -> f64;
+}
diff --git a/src/formula/node/mod.rs b/src/formula/node/mod.rs
index 90c22b9..6e53a7a 100644
--- a/src/formula/node/mod.rs
+++ b/src/formula/node/mod.rs
@@ -1,16 +1,27 @@
 use crate::formula::node::graph_constructor::GraphBuilder;
 
+mod function;
 mod graph_constructor;
 
 pub struct Node {
     children: Vec<Node>,
+    function: fn(Vec<f64>) -> f64,
 }
 
 impl Node {
     pub fn empty() -> Self {
-        Node { children: vec![] }
+        Node {
+            children: vec![],
+            function: |_| 0f64,
+        }
     }
     pub fn build() -> GraphBuilder {
         GraphBuilder
     }
+    pub fn get_value(&self) -> f64 {
+        if self.children.len() == 0 {
+            0f64
+        }
+        for node in self.children {}
+    }
 }