diff --git a/src/formula.rs b/src/formula.rs
index f2700da..179fe40 100644
--- a/src/formula.rs
+++ b/src/formula.rs
@@ -1,4 +1,5 @@
 use crate::node::Node;
+use crate::node::node_modifier::NodeModifier;
 
 #[derive(Clone)]
 pub struct Formula {
@@ -10,7 +11,7 @@ impl Formula {
         let mut formula = Self {
             tree: Node::empty(),
         };
-        formula.tree.modify_tree().add_node(Node::variable());
+        formula.tree.modify_node().add_node(Node::variable());
 
         formula
     }
@@ -22,4 +23,29 @@ impl Formula {
         outputs
     }
     pub fn mutate(&mut self) {}
+
+    pub fn modify_tree(&mut self) -> NodeModifier {
+        self.tree.modify_tree()
+    }
+    pub fn display_tree(&self) {
+        self.display_recursion(0, vec![&self.tree]);
+    }
+    fn display_recursion(&self, indent_level: u8, nodes: Vec<&Node>) {
+        for node in nodes {
+            if indent_level != 0 {
+                for _ in 0..(indent_level) {
+                    print!("|\t");
+                }
+            }
+            println!("{node}");
+            if node.children.len() == 0 {
+                continue;
+            }
+            let mut next_nodes: Vec<&Node> = Vec::new();
+            for node in &node.children {
+                next_nodes.push(node);
+            }
+            self.display_recursion(indent_level + 1, next_nodes);
+        }
+    }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 52cf965..ac0c947 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,2 +1,4 @@
-mod formula;
-mod node;
+pub mod formula;
+pub mod node;
+#[cfg(test)]
+mod tests;
diff --git a/src/main.rs b/src/main.rs
index 9e18c4f..e1b67d2 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,9 +1,22 @@
-use crate::formula::Formula;
+use crate::{formula::Formula, node::Node};
 
 mod formula;
 mod node;
+#[cfg(test)]
+mod tests;
 
 fn main() {
-    let formula = Formula::new();
-    dbg!(formula.run(vec![1f64]));
+    let mut formula = Formula::new();
+    formula
+        .modify_tree()
+        .insert_node(Node::function(|inputs| inputs.iter().sum(), Some(2)), None);
+    if let Err(x) = formula
+        .modify_tree()
+        .go_down(0)
+        .add_node(Node::number(1f64))
+    {
+        println!("{x}");
+    }
+    formula.display_tree();
+    let results = formula.run(vec![0f64, 1f64, 2f64, 3f64, 4f64, 5f64]);
 }
diff --git a/src/node/mod.rs b/src/node/mod.rs
index 34dcad1..06b3746 100644
--- a/src/node/mod.rs
+++ b/src/node/mod.rs
@@ -1,15 +1,17 @@
+use std::fmt;
+
 use crate::node::node_modifier::NodeModifier;
 use handler::*;
 
 mod functions;
 mod handler;
-mod node_modifier;
+pub mod node_modifier;
 
 #[derive(Clone)]
 pub struct Node {
-    children: Vec<Node>,
-    handler: NodeHandler,
-    max_children_count: Option<usize>,
+    pub(crate) children: Vec<Node>,
+    pub(crate) handler: NodeHandler,
+    pub(crate) max_children_count: Option<usize>,
 }
 
 impl Node {
@@ -63,7 +65,25 @@ impl Node {
 
         return self.handler.run(inputs, passed_x);
     }
-    pub fn modify_tree(&mut self) -> NodeModifier {
+    pub fn modify_node(&mut self) -> NodeModifier {
         NodeModifier::from_random(self, None)
     }
+    pub fn modify_tree(&mut self) -> NodeModifier {
+        NodeModifier::from_node(self, None)
+    }
+}
+
+impl fmt::Display for Node {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "{}",
+            match self.handler {
+                NodeHandler::Number { number } => number.to_string(),
+                NodeHandler::Function { function } => "Function".to_string(),
+                NodeHandler::Variable => "X".to_string(),
+                NodeHandler::Empty => "Empty".to_string(),
+            }
+        )
+    }
 }
diff --git a/src/node/node_modifier.rs b/src/node/node_modifier.rs
index b2cd066..655a104 100644
--- a/src/node/node_modifier.rs
+++ b/src/node/node_modifier.rs
@@ -1,3 +1,5 @@
+use std::fmt;
+
 use crate::node::Node;
 use crate::node::NodeHandler;
 use crate::node::functions;
@@ -9,6 +11,17 @@ const TYPE_CHANGE_PROBABILITY: u8 = 10;
 pub enum NodeManipulationError {
     TooMuchChildren(Node),
 }
+impl fmt::Display for NodeManipulationError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "{}",
+            match self {
+                NodeManipulationError::TooMuchChildren(node) => "Too much children",
+            }
+        )
+    }
+}
 
 pub struct NodeModifier<'a> {
     picked_node: &'a mut Node,
@@ -71,6 +84,10 @@ impl<'a> NodeModifier<'a> {
             number_mutation_pool: pools.1,
         }
     }
+    pub fn go_down(mut self, to_child: usize) -> Self {
+        self.picked_node = &mut self.picked_node.children[to_child];
+        self
+    }
 
     pub fn add_node(&mut self, node: Node) -> Result<(), NodeManipulationError> {
         if let Some(x) = self.picked_node.max_children_count {
diff --git a/src/tests.rs b/src/tests.rs
new file mode 100644
index 0000000..8a16cb1
--- /dev/null
+++ b/src/tests.rs
@@ -0,0 +1,38 @@
+use crate::{formula::Formula, node::Node};
+
+#[test]
+fn test_node_variable() {
+    let formula = Formula::new();
+    let results = formula.run(vec![0f64, 1f64, 2f64, 3f64, 4f64, 5f64]);
+    assert_eq!(results, vec![0f64, 1f64, 2f64, 3f64, 4f64, 5f64])
+}
+
+#[test]
+fn test_plus_one() {
+    let mut formula = Formula::new();
+    assert!(
+        formula
+            .modify_tree()
+            .insert_node(Node::function(|inputs| inputs[0] + 1f64, Some(1)), None)
+            .is_err()
+            == false
+    );
+    let results = formula.run(vec![0f64, 1f64, 2f64, 3f64, 4f64, 5f64]);
+    assert_eq!(results, vec![1f64, 2f64, 3f64, 4f64, 5f64, 6f64])
+}
+
+#[test]
+fn test_branch_sum() {
+    let mut formula = Formula::new();
+    assert!(
+        formula
+            .modify_tree()
+            .insert_node(Node::function(|inputs| inputs.iter().sum(), Some(2)), None)
+            .is_err()
+            == false
+    );
+    assert!(formula.modify_tree().add_node(Node::number(1f64)).is_err() == false);
+    formula.display_tree();
+    let results = formula.run(vec![0f64, 1f64, 2f64, 3f64, 4f64, 5f64]);
+    assert_eq!(results, vec![1f64, 2f64, 3f64, 4f64, 5f64, 6f64])
+}