[boost] [Fusion] fold takes state by cref. Why ?

Why is the State passed by const reference ? I have a couple of use case (like using fold to access the element (i0,...,in) of a fusion sequence of pointer to ... pointer to T array) where I need to be able to discriminate the constness of the State to forward to the proper PFO overload of operator(). Is there any intrinsic stuff I am missing ?