code: def _area(sma, eps, phi, r):
      
  
      aux = r * math.cos(phi) / sma
      signal = aux / abs(aux)
      if abs(aux) >= 1.:
          aux = signal
      return abs(sma**2 * (1.-eps) / 2. * math.acos(aux))
docs: Compute elliptical sector area.