Hypoplastic rate independent and non-linearity

Dear all
I’m studying hypo-plastic constitutive law from beginning concepts
I traced concepts successfully until i faced **rate independent topic**
i read this topic in the prof. Wolfgangs paper (hypo-plasticity for beginners)
he said that “Rate-independence means that the gradient of the curve (stiffness) does not depend on the strain rate, Thus the stress rate must be precisely twice as large for a double strain rate” and in the fallowing he said that “the constitutive law may not have terms like (strain rate^2) (strain rate square )”
as you know the hypo-plastic constitutive law is non-linear in strain rate
my question is that how hypo-plastic constitutive law can fulfill the non-linearity (in strain-rate) with no term like (strain rate^2)