diff --git a/rangeset.py b/rangeset.py
new file mode 100755
index 00000000..dd5952f1
--- /dev/null
+++ b/rangeset.py
@@ -0,0 +1,34 @@
+#!/bin/python3
+import unittest
+
+class RangeSet:
+	def __init__(self, rangespec):
+		self.ranges = self.parse_spec(rangespec)
+	def __contains__(self, num):
+		return any(num in r for r in self.ranges)
+	def parse_spec(self, spec: str) -> list[range]:
+		result = []
+		parts = spec.split(',')
+		parts = map(str.strip, parts)
+		for part in parts:
+			if '-' in part:
+				start, end = part.split('-')
+				result.append(range(int(start), int(end)+1))
+			else:
+				n = int(part)
+				result.append(range(n, n+1))
+		return result
+	# TODO: simplification of inner ranges
+	# TODO: Set operations: union, intersection, difference?
+	# TODO: Comparison to classic range objects (only to check that it represents the same numbers)
+
+class TestRangeSetBasic(unittest.TestCase):
+	def test_number_sequence(self):
+		rs = RangeSet('1,3,4,5')
+		for x in [1,3,4,5]:
+			self.assertIn(x, rs)
+		self.assertNotIn(2, rs)
+
+if __name__ == '__main__':
+	print('This is a library. Runing the testsuite instead.')
+	unittest.main()