diff --git a/scalcs/__init__.py b/scalcs/__init__.py
new file mode 100644
index 0000000..437c72b
--- /dev/null
+++ b/scalcs/__init__.py
@@ -0,0 +1,11 @@
+"""SCALCS: Q-matrix calculations for single ion channel kinetics.
+
+This file makes scalcs a regular package. Without it, Python treats scalcs as
+a namespace package and merges every directory on the path that holds a scalcs
+folder, so a module missing from the copy being tested is quietly imported from
+another copy - an editable install, say. That once made a check report success
+against one version of SCALCS while running modules from another.
+
+Keep it light: importing scalcs should import nothing but the version.
+"""
+from scalcs.version import __version__  # noqa: F401
diff --git a/tests/test_package.py b/tests/test_package.py
new file mode 100644
index 0000000..1d71da0
--- /dev/null
+++ b/tests/test_package.py
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+"""scalcs must be a regular package, not a namespace package.
+
+Without scalcs/__init__.py, Python treats scalcs as a namespace package and
+merges every directory on the path that contains a scalcs folder. A test
+pointed at one copy of SCALCS - an older release, a branch under review - then
+silently imports any module that copy lacks from another copy on the path: an
+installed release in site-packages, a checkout on PYTHONPATH.
+
+With __init__.py the first scalcs found on the path is used whole, and a
+module it lacks is an ImportError.
+
+What this does NOT prevent: an editable install (pip install -e) registers its
+own finder, which resolves any scalcs.* name directly to its own source tree
+and ignores which copy of scalcs was imported. It is consulted after the normal
+path search, so it still fills in any module the first copy lacks. That is what
+produced the false pass during the reproduction of Colquhoun & Lape (2012), and
+nothing inside the package can stop it. To test one copy of SCALCS while an
+editable install of another exists, use a clean environment, or remove that
+finder from sys.meta_path and then check every imported module's __file__.
+"""
+import os
+import shutil
+import subprocess
+import sys
+
+import pytest
+
+import scalcs
+from scalcs import version
+
+
+def test_scalcs_is_a_regular_package():
+    assert scalcs.__file__ is not None
+    assert os.path.basename(scalcs.__file__) == '__init__.py'
+    assert type(scalcs.__path__) is list         # a namespace package's is not
+
+
+#: Run in a subprocess with two copies of scalcs on the path. Any editable-
+#: install finder is removed first, so that only the path search is tested -
+#: see the module docstring for why that finder is out of the package's reach.
+_TWO_COPIES = '''
+import sys
+STANDARD = ('_frozen_importlib', '_frozen_importlib_external')
+sys.meta_path = [f for f in sys.meta_path
+                 if getattr(type(f), '__module__', '').startswith(STANDARD)
+                 or getattr(f, '__module__', '').startswith(STANDARD)]
+for name in [m for m in sys.modules if m.split('.')[0] == 'scalcs']:
+    del sys.modules[name]
+import scalcs.qmatlib
+print('qmatlib came from', scalcs.qmatlib.__file__)
+'''
+
+
+def test_another_copy_on_the_path_cannot_fill_in_modules(tmp_path):
+    """Two copies of scalcs on the path: first a copy laid out like this one
+    but without qmatlib, then the real one. Importing scalcs.qmatlib must
+    fail, rather than quietly come from the second copy."""
+    real_package = os.path.dirname(os.path.abspath(version.__file__))
+    real_root = os.path.dirname(real_package)
+    first = tmp_path / 'scalcs'
+    first.mkdir()
+    for name in ('__init__.py', 'version.py'):
+        if os.path.exists(os.path.join(real_package, name)):
+            shutil.copy(os.path.join(real_package, name), first / name)
+
+    env = dict(os.environ,
+               PYTHONPATH=os.pathsep.join([str(tmp_path), real_root]))
+    run = subprocess.run([sys.executable, '-c', _TWO_COPIES], cwd=tmp_path,
+                         env=env, capture_output=True, text=True)
+    assert run.returncode != 0, run.stdout
+    assert "No module named 'scalcs.qmatlib'" in run.stderr, run.stderr
+
+
+def test_the_version_is_on_the_package():
+    assert scalcs.__version__ == version.__version__
+
+
+def test_importing_the_package_imports_nothing_heavy():
+    """`import scalcs` must stay cheap: scalcs.schemes, for one, relies on
+    nothing importing matplotlib behind its back."""
+    code = ('import sys, scalcs; '
+            'print(sorted(m for m in ("numpy", "scipy", "matplotlib") '
+            'if m in sys.modules))')
+    run = subprocess.run([sys.executable, '-c', code], capture_output=True,
+                         text=True, check=True)
+    assert run.stdout.strip() == '[]'