Small simple structure

src/formula/mod.rs

@@ -0,0 +1,18 @@
+use crate::formula::node::Node;
+
+pub mod node;
+
+pub struct Formula {
+    root: node::Node,
+}
+
+impl Formula {
+    pub fn new(optional_root: Option<node::Node>) -> Self {
+        match optional_root {
+            Some(root) => Formula { root },
+            None => Formula {
+                root: node::Node::empty(),
+            },
+        }
+    }
+}

src/formula/node/graph_constructor.rs

@@ -0,0 +1 @@
+pub struct GraphBuilder;

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
+    }
+}

src/lib.rs

@@ -0,0 +1 @@
+mod formula;