cotanSort :: [Point] -> [Point]
cotanSort ps = initialPoint : sortBy cotanCompare remainingPoints
    where
      initialSorting = initialSort ps
      initialPoint = head initialSorting
      remainingPoints = tail initialSorting
      (pivotx,pivoty) = initialPoint
      cotanCompare (px,py) (qx,qy)
          | pCalc > qCalc = GT
          | pCalc < qCalc  = LT
          | py < qy       = GT
          | px > qx       = GT
          | otherwise     = EQ
          where
            pCalc = (py-pivoty)*(qx-pivotx)
            qCalc = (qy-pivoty)*(px-pivotx)