def transform_y(f_x_y, xf_and_dxf_dyprime):
	"""Transform f_x_y(x,y) into f_x_yprime(x,yprime)
	xf is the inverse transformation: from yprime to y
	y = xf(yprime, *xf_params)
	dxf_dyprime is its derivative with respect to yprime"""
	def f_x_yprime(x,yprime,p):
		xf, dxf_dyprime = xf_and_dxf_dyprime(yprime, p)
		return f_x_y(x, xf, p) * dxf_dyprime
	return f_x_yprime