diff --git a/connect4.py b/connect4.py
new file mode 100644
index 0000000..2c8be26
--- /dev/null
+++ b/connect4.py
@@ -0,0 +1,74 @@
+from __future__ import annotations
+
+import typing as tp
+from dataclasses import dataclass
+from functools import cache
+
+Player = tp.Literal["X", "O"]
+Winner = Player | tp.Literal["draw"]
+Space = Player | tp.Literal[" "]
+Board = tuple[Space, ...]
+Score = int
+
+C4_COLS = 7
+C4_ROWS = 6
+
+
+@dataclass(frozen=True)
+class C4GameState:
+    player: Player
+    board: Board
+
+    def __str__(self) -> str:
+        b = self.board
+        return f"───┼───┼───┼───┼───┼───┼───\n".join(
+            f" {b[i+0]} │ {b[i+1]} │ {b[i+2]} │ {b[i+3]} │ {b[i+4]} │ {b[i+5]} │ {b[i+6]}\n"
+            for i in range(0, C4_COLS * C4_ROWS, C4_COLS)
+        )
+
+
+@dataclass(frozen=True)
+class Move:
+    col: int
+
+    def __str__(self) -> str:
+        return str(self.col + 1)
+
+
+@cache
+def get_valid_moves(state: C4GameState) -> tp.Iterable[Move]:
+    # NOTE: a slot must not be full if its top row is empty
+    return tuple(Move(col) for col in range(C4_COLS) if state.board[col] == " ")
+
+
+@cache
+def apply_move(state: C4GameState, move: Move) -> C4GameState:
+    new_player = "X" if state.player == "O" else "O"
+
+    # B)
+    for i in range(1, C4_ROWS):
+        idx_above = (i - 1) * C4_COLS + move.col
+        idx_below = (i - 0) * C4_COLS + move.col
+        if state.board[idx_below] != " ":
+            idx = idx_above
+            break
+    else:
+        # bottom
+        idx = (C4_ROWS - 1) * C4_COLS + move.col
+
+    new_board = list(state.board)
+    new_board[idx] = state.player
+    new_board = tuple(new_board)
+
+    return C4GameState(player=new_player, board=new_board)
+
+@cache
+def get_winner(state: C4GameState) -> Winner | None:
+    pass
+
+START = C4GameState(player="X", board=(" ",) * C4_COLS * C4_ROWS)
+
+if __name__ == "__main__":
+    print(START)
+
+    print(apply_move(START, Move(col=6)))
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..cbc3a54
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1 @@
+pdbp
diff --git a/tictactoe.py b/tictactoe.py
index f565e76..0e7d3b5 100644
--- a/tictactoe.py
+++ b/tictactoe.py
@@ -33,7 +33,7 @@ class TTTGameState:
     player: Player
     board: Board
 
-    def __str__(self):
+    def __str__(self) -> str:
         b = self.board
         return (
             f" {b[0]} │ {b[1]} │ {b[2]}\n"
@@ -52,14 +52,14 @@ class Move:
         return str(self.position + 1)
 
 
-# @cache
+@cache
 def get_valid_moves(state: TTTGameState) -> tp.Iterable[Move]:
     return tuple(
         Move(position=i) for i, square in enumerate(state.board) if square == " "
     )
 
 
-# @cache
+@cache
 def apply_move(state: TTTGameState, move: Move) -> TTTGameState:
     new_player = "X" if state.player == "O" else "O"
 
@@ -70,7 +70,7 @@ def apply_move(state: TTTGameState, move: Move) -> TTTGameState:
     return TTTGameState(player=new_player, board=new_board)
 
 
-# @cache
+@cache
 def get_winner(state: TTTGameState) -> Winner | None:
     # abc
     # def
@@ -103,18 +103,18 @@ def get_winner(state: TTTGameState) -> Winner | None:
     if not any(square == " " for square in state.board):
         return "draw"
 
-    # no winner
+    # no winner yet
     return None
 
 
-# @cache
+@cache
 def get_next_states(state: TTTGameState) -> tuple[tuple[Move, TTTGameState], ...]:
     assert get_winner(state) is None, "should not be called if game ended"
     return tuple((move, apply_move(state, move)) for move in get_valid_moves(state))
 
 
-# @cache
-@util.count_calls
+@cache
+# @util.count_calls
 def get_score(target: Player, state: TTTGameState) -> Score:
     winner = get_winner(state)
     if winner == target:
@@ -132,7 +132,7 @@ def get_score(target: Player, state: TTTGameState) -> Score:
     return score
 
 
-# @cache
+@cache
 def get_scored_moves(
     target: Player, state: TTTGameState
 ) -> tuple[tuple[Score, Move], ...]: