filter and other views transform the sequence type

[Robert Ramey]

code such as the following doesn't work. That is, none of the views can produce a set type sequence. Note this is diffferent from fusion so it is possible. I also realize that this is non-trivial enhancement. Oh well.

    struct a {};
    struct b {};
    struct c {};
    struct d {};
    typedef boost::mpl::set<a, b, c> s1;
    BOOST_CONCEPT_ASSERT(( boost::mpl::AssociativeSequence<s1> ));
    typedef boost::mpl::set<c, d> s2;
    BOOST_CONCEPT_ASSERT(( boost::mpl::AssociativeSequence<s2> ));
    typedef boost::mpl::joint_view<s1, s2>::type sdiff;
    BOOST_CONCEPT_ASSERT(( boost::mpl::AssociativeSequence<sdiff> ));

I know it's correct as is - that is it works according to they it's described in the manual. But it was hard to find my mistake.