diff --git a/notes-and-examples/2026.05.13/homework.py b/notes-and-examples/2026.05.13/homework.py
index e69de29..19d76c2 100644
--- a/notes-and-examples/2026.05.13/homework.py
+++ b/notes-and-examples/2026.05.13/homework.py
@@ -0,0 +1,49 @@
+# Also do the 2026.05.06 homework
+#
+# Implement insertion, removal and search in a binary search tree.
+# Properties of binary search trees:
+# - Each node has at most two children
+# - The left subtree of a node contains only nodes with values less than the node's value
+# - The right subtree of a node contains only nodes with values greater than the node's value
+#
+# Example of what such a tree would look like:
+#       10
+#      /  \
+#     5   15
+#    / \    \
+#   3   7   20
+#
+# Example of an *invalid* tree:
+#       10
+#      /  \
+#     5   15
+#    / \    \
+#   3   7   20
+#    \
+#     12
+#
+# Don't worry about balancing the tree for now, we will cover that later
+
+
+class Node:
+    def __init__(self, value):
+        self.value = value
+        self.left = None
+        self.right = None
+
+
+class BinarySearchTree:
+    def __init__(self):
+        self.root = None
+
+    def insert(self, value):
+        # TODO
+        pass
+
+    def remove(self, value):
+        # TODO
+        pass
+
+    def search(self, value):
+        # TODO
+        pass
diff --git a/notes-and-examples/2026.05.13/trees.py b/notes-and-examples/2026.05.13/trees.py
new file mode 100644
index 0000000..ffadd5d
--- /dev/null
+++ b/notes-and-examples/2026.05.13/trees.py
@@ -0,0 +1,121 @@
+# Example: expression tree + evaluation
+
+from enum import Enum
+
+
+class TokenType(Enum):
+    NUMBER = 1
+    OPERATOR = 2
+
+
+# either store an operator (+, -, *, /) or a number
+class Token:
+    def __init__(self, type: TokenType, value: str):
+        self.type = type
+        self.value = value
+
+
+# each individual element in the tree
+class Node:
+    left = None
+    right = None
+
+    def __init__(self, token: Token, left=None, right=None):
+        self.token = token
+        self.left = left
+        self.right = right
+
+
+# holder for the tree
+class ExpressionTree:
+    def __init__(self, root: Node | None):
+        self.root = root
+
+
+def evaluate(tree: ExpressionTree) -> int:
+    return _evaluate(tree.root)
+
+
+# evaluate a tree recursively starting from the node
+def _evaluate(node: Node | None) -> int:
+    if node is None:
+        return 0
+
+    if node.token.type == TokenType.NUMBER:
+        return int(node.token.value)
+
+    elif node.token.type == TokenType.OPERATOR:
+        if node.left is None or node.right is None:
+            raise ValueError(f"Operator {node.token.value} requires two operands")
+
+        left = _evaluate(node.left)
+        right = _evaluate(node.right)
+
+        if node.token.value == "+":
+            return left + right
+        elif node.token.value == "-":
+            return left - right
+        elif node.token.value == "*":
+            return left * right
+        elif node.token.value == "/":
+            return left // right
+
+    raise ValueError(f"Unknown operator: {node.token.value}")
+
+
+# test cases
+test_cases = [
+    (ExpressionTree(Node(Token(TokenType.NUMBER, "5"))), 5),
+    (
+        ExpressionTree(
+            Node(
+                Token(TokenType.OPERATOR, "+"),
+                Node(Token(TokenType.NUMBER, "3")),
+                Node(Token(TokenType.NUMBER, "2")),
+            )
+        ),
+        5,
+    ),
+    (
+        ExpressionTree(
+            Node(
+                Token(TokenType.OPERATOR, "+"),
+                Node(Token(TokenType.NUMBER, "3")),
+                Node(
+                    Token(TokenType.OPERATOR, "*"),
+                    Node(Token(TokenType.NUMBER, "2")),
+                    Node(Token(TokenType.NUMBER, "3")),
+                ),
+            )
+        ),
+        9,
+    ),
+    (
+        ExpressionTree(
+            Node(
+                Token(TokenType.OPERATOR, "+"),
+                Node(
+                    Token(TokenType.OPERATOR, "-"),
+                    Node(Token(TokenType.NUMBER, "3")),
+                    Node(Token(TokenType.NUMBER, "2")),
+                ),
+                Node(
+                    Token(TokenType.OPERATOR, "*"),
+                    Node(Token(TokenType.NUMBER, "2")),
+                    Node(Token(TokenType.NUMBER, "3")),
+                ),
+            )
+        ),
+        7,
+    ),
+]
+
+
+def test_evaluate() -> None:
+    for tree, expected in test_cases:
+        evaluation = evaluate(tree) == expected
+        print(f"Test case: evaluate({tree}) == {expected} -> {evaluation}")
+
+
+if __name__ == "__main__":
+    test_evaluate()