λxs.λys.
{
  Σ ← [(+)/x];
  n ⟜ ℝ(:xs);
  xbar ⟜ (Σ xs)%n; ybar ← (Σ ys)%n;
  xy ← Σ ((*)`xs ys); x2 ← Σ ((^2)'xs);
  denom ← (x2-(n*(xbar^2)));
  (xy-(n*xbar*ybar))%denom
}