What is a core constant expression in the C++11 Standard?

There are 11 references to the expression core constant expression in the latest draft of the C++11 Standard (N3690), and none of them defines what this entity is.

One can also find that the expression core constant expression is pretty well defined here , basically in the same terms that the Standard uses to define the expression conditional-expression.

Therefore, I would like to get some input on this issue, which seems to me, to be wrong in the Standard.

Now, assuming the definition in cppreference is correct I would also like to know why the following snippet compiles in Coliru and in Ideone, despite item (10) in the alluded definition?

#include <iostream>

int main()
{
    const double x = 2.;
    constexpr double y = x;
    std::cout << y << std::endl;
}

I'm specifically thinking in terms of the lvalue to rvalue implicit conversion of variable x in the expression constexpr double y = x;, which is not covered by any of the clauses (a), (b) and (c) in item (10) referred above.

Thanks for the help.