Readable concept in Range v3

What are the minimum syntaxis requirements for an iterator to be Readable according to Range v3? Can that be written a set of statemens-that-must-compile.

I have an iterator It with which I can do the basic stuff (what I would call "readable"), yet it doesn't pass the concept check:

    It i; // ok
    typename It::value_type val = *i; // ok
    typename It::reference ref = *i;   // ok
    static_assert( ranges::Readable<multi::array<double, 2>::iterator>{} ); // assert error

What other constructs involving i I can write that it will make obvious that It is not Readable?