diff --git a/debugger_protocol/arg/__init__.py b/debugger_protocol/arg/__init__.py
new file mode 100644
index 00000000..fc4abbf4
--- /dev/null
+++ b/debugger_protocol/arg/__init__.py
@@ -0,0 +1 @@
+from ._common import NOT_SET, ANY  # noqa
diff --git a/debugger_protocol/arg/_common.py b/debugger_protocol/arg/_common.py
new file mode 100644
index 00000000..c36af8e7
--- /dev/null
+++ b/debugger_protocol/arg/_common.py
@@ -0,0 +1,17 @@
+
+def sentinel(name):
+    """Return a named value to use as a sentinel."""
+    class Sentinel(object):
+        def __repr__(self):
+            return name
+
+    return Sentinel()
+
+
+# NOT_SET indicates that an arg was not provided.
+NOT_SET = sentinel('NOT_SET')
+
+# ANY is a datatype surrogate indicating that any value is okay.
+ANY = sentinel('ANY')
+
+SIMPLE_TYPES = {None, bool, int, str}