Accelerate the convergence of the a series
	using Aitken's delta-squared process (SCIP calls it Euler).

def Aitken(s):
	"""Accelerate the convergence of the a series
	using Aitken's delta-squared process (SCIP calls it Euler).
	"""
	def accel():
		s0, s1, s2 = s >> item[:3]
		while 1:
			yield s2 - (s2 - s1)**2 / (s0 - 2*s1 + s2)
			s0, s1, s2 = s1, s2, next(s)
	return accel()