diff --git a/shell.nix b/shell.nix
index 95591a5..e2f91bd 100644
--- a/shell.nix
+++ b/shell.nix
@@ -2,13 +2,17 @@
 
 pkgs.mkShell {
   packages = [
-    (pkgs.python3.withPackages (pythonPackages: [
-      pythonPackages.simpy
-      pythonPackages.prompt-toolkit
+    # Python + third-party deps
+    (pkgs.python313.withPackages (ps: [
+      ps.simpy
+      ps.prompt-toolkit
     ]))
 
-    pkgs.basedpyright
-    pkgs.codex-acp
-    pkgs.clang
+    # Frontend toolchain
+    pkgs.nodejs_22
   ];
+
+  shellHook = ''
+    echo "simpy-terns: python $(python --version 2>&1 | cut -d' ' -f2), node $(node --version)"
+  '';
 }
diff --git a/tern.py b/tern.py
new file mode 100644
index 0000000..af2a1ba
--- /dev/null
+++ b/tern.py
@@ -0,0 +1,26 @@
+from enum import Enum
+from typing import override
+
+
+class Tern(Enum):
+    N = -1
+    U = 0
+    Y = 1
+
+    @override
+    def __str__(self):
+        if self.value == -1:
+            return "0"
+        if self.value == 1:
+            return "1"
+        return "N"
+
+    @classmethod
+    def from_string(cls, s: str) -> "Tern":
+        if s == "N":
+            return Tern.U
+        if s == "0":
+            return Tern.N
+        if s == "1":
+            return Tern.Y
+        return cls[s]