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diff --git a/challenge-220/roger-bell-west/python/ch-2.py b/challenge-220/roger-bell-west/python/ch-2.py
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+++ b/challenge-220/roger-bell-west/python/ch-2.py
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+#! /usr/bin/python3
+
+import collections
+from itertools import permutations, islice
+
+# https://docs.python.org/3/library/itertools.html
+def sliding_window(iterable, n):
+    # sliding_window('ABCDEFG', 4) --> ABCD BCDE CDEF DEFG
+    it = iter(iterable)
+    window = collections.deque(islice(it, n), maxlen=n)
+    if len(window) == n:
+        yield tuple(window)
+    for x in it:
+        window.append(x)
+        yield tuple(window)
+
+def squared(a):
+  return a * a
+
+def squareful(lst):
+  results = set()
+  squares = set()
+  for la in permutations(lst):
+    squareful = True
+    for a in sliding_window(la, 2):
+      cs = a[0] + a[1]
+      mx = squared(len(squares))
+      while cs > mx:
+        mx = squared(len(squares) + 1)
+        squares.add(mx)
+      if cs not in squares:
+        squareful = False
+        break
+    if squareful:
+      results.add(tuple(la))
+  o = list(results)
+  o.sort()
+  return [ list(x) for x in o ]
+
+
+import unittest
+
+class TestSquareful(unittest.TestCase):
+
+  def test_ex1(self):
+    self.assertEqual(squareful([1, 17, 8]), [[1, 8, 17], [17, 8, 1]], 'example 1')
+
+  def test_ex2(self):
+    self.assertEqual(squareful([2, 2, 2]), [[2, 2, 2]], 'example 2')
+
+unittest.main()