diff --git a/pyproject.toml b/pyproject.toml
index 4fb0ee7..e0aad04 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -41,3 +41,4 @@ formats = "ipynb,py"
 exclude = ".venv"
 venvPath = "."
 venv = ".venv"
+reportMissingImports = false
diff --git a/src/__init__.py b/src/__init__.py
index 00e3147..e69de29 100644
--- a/src/__init__.py
+++ b/src/__init__.py
@@ -1,3 +0,0 @@
-#! /usr/bin/env python3
-# Remove this file if you don't have a main.py.
-from .main import *
diff --git a/src/eqx.py b/src/eqx.py
new file mode 100644
index 0000000..41e63d8
--- /dev/null
+++ b/src/eqx.py
@@ -0,0 +1,44 @@
+#! /usr/bin/env python3
+
+from typing import Callable, Optional
+
+import equinox as eqx
+
+from .fn import compose
+
+
+def create_hook(
+    fwd_pre: Callable = lambda *arg, **kwarg: None,
+    fwd_post: Callable = lambda *arg, **kwarg: None,
+    bwd_pre: Callable = lambda *arg, **kwarg: None,
+    bwd_post: Callable = lambda *arg, **kwarg: None,
+) -> Callable:
+    def _create_hook(node: eqx.Module) -> eqx.Module:
+        node_call = type(node).__call__
+
+        @eqx.filter_custom_jvp
+        def fwd(hook, *args, **kwargs):
+            fwd_pre(*args, **kwargs)
+            out = node_call(hook, *args, **kwargs)
+            fwd_post(out)
+            return out
+
+        @fwd.def_jvp
+        def bwd(primals, tangents):
+            bwd_pre(*primals, *tangents)
+            primals_out, tangents_out = eqx.filter_jvp(
+                node_call, primals, tangents
+            )
+            bwd_post(primals_out, tangents_out)
+            return primals_out, tangents_out
+
+        class Hook(type(node)):
+            def __init__(self, node):
+                self.__dict__.update(node.__dict__)
+
+            def __call__(self, *args, **kwargs):
+                return fwd(self, *args, **kwargs)
+
+        return Hook(node)
+
+    return _create_hook
diff --git a/src/fn.py b/src/fn.py
new file mode 100644
index 0000000..bb3e490
--- /dev/null
+++ b/src/fn.py
@@ -0,0 +1,26 @@
+#! /usr/bin/env python3
+# vim:fenc=utf-8
+
+from functools import reduce
+
+
+
+def compose(*func):
+    """Create a single function out of multiple functions."""
+
+    def compose_(f, g):
+        return lambda *args, **kwargs: f(g(*args, **kwargs))
+
+    return reduce(compose_, func, lambda out: out)
+if __name__ == "__main__":
+
+    def fn0(x):
+        print("fn0", x + 1)
+        return x + 1
+
+    def fn1(x):
+        print("fn1", -x)
+        return -x
+
+    fn2 = compose(fn1, fn0)
+    assert fn2(5) == -6