-- | The sign of the endpoint shall be the same for all points except
-- for the first point
prop_ptsShallHaveEqualSign :: DottedLine -> Bool
prop_ptsShallHaveEqualSign (DottedLine start end seg (p:ps)) =
  foldl (checkSign end) True ps
prop_ptsShallHaveEqualSign :: DottedLine -> Bool
prop_ptsShallHaveEqualSign (DottedLine start end seg (p:ps)) =
  foldl (checkSign end) True ps
prop_ptsShallHaveEqualSign (DottedLine start end seg (p:ps)) =
  foldl (checkSign end) True ps