spirit::karma specialization for std::optional (C++17)

Hello,

I'm working in a spirit::karma specialization for std::optional (C++17). I
have this code for now:

//-----------------------------------

namespace boost
{
namespace spirit
{
namespace traits
{

// This handles std::optional attributes
template <typename Attribute, typename Exposed>
struct extract_from_attribute<std::experimental::optional<Attribute>,
Exposed>
{
    typedef Attribute const& type;
    template <typename Context>
    static type call(std::experimental::optional<Attribute> const& attr,
Context& ctx)
    {
        return attr.value();
    }
};

template <typename Attribute, typename Exposed>
struct extract_from_attribute<std::experimental::optional<Attribute const>,
Exposed>
{
    typedef Attribute const& type;
    template <typename Context>
    static type call(std::experimental::optional<Attribute const> const&
attr, Context& ctx)
    {
        return attr.value();
    }
};

}
}
}

//-----------------------------------

This works good to extract the value, but, I need to handle the calls if
the optional have, or not, a value. I tried to understand how karma handle
this for boost::optional, but I'm stuck with that. Any idea?

Thanks!!

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