offset.apple

-- A0 and C0 in Kuhl+Giardina
λxs.λys.
  { sum ← [(+)/x]
  ; tieSelf ← [({.x)⊳x]; Δ ← [(-)\~(tieSelf x)]
  ; dxs ⟜ Δ xs; dys ⟜ Δ ys
  ; dts ⟜ [√(x^2+y^2)]`dxs dys
  ; pxs ← (+)Λ dxs; pys ← (+)Λ dys; pts ⟜ (+)Λ dts
  ; dtss ⟜ (-)\~((^2)'(0<|pts))
  ; T ⟜}. pts
  ; 𝜉 ← (-)`pxs ((*)`((%)`dxs dts) pts)
  ; 𝛿 ← (-)`pys ((*)`((%)`dys dts) pts)
  ; A ← ((sum ((*)`((%)`dxs dts) dtss))%2 + (sum ((*)`𝜉 dts)))%T
  ; C ← ((sum ((*)`((%)`dys dts) dtss))%2 + (sum ((*)`𝛿 dts)))%T
  ; (A,C)
  }