Small simple structure

src/formula/node/mod.rs

@@ -0,0 +1,16 @@
+use crate::formula::node::graph_constructor::GraphBuilder;
+
+mod graph_constructor;
+
+pub struct Node {
+    children: Vec<Node>,
+}
+
+impl Node {
+    pub fn empty() -> Self {
+        Node { children: vec![] }
+    }
+    pub fn build() -> GraphBuilder {
+        GraphBuilder
+    }
+}