Hello Boost Users, I've encountered a problem while using mpl::fold with mpl::set I've defined a template to check if one sequence contains all elements of the other: namespace mpl = boost::mpl; template<class A, class B> struct subset : mpl::fold< A, mpl::true_, mpl::and_<mpl::_1, mpl::contains<B, mpl::_2> > > {}; It works fine with lists and vectors but when first template parameter (A) is set or set_c it fails to compile. Did I miss something? Why is it impossible to iterate through a set via fold? p.s. tested on gcc and clang under linux. p.p.s. compilation error is quite huge, the main message is /usr/local/include/boost/mpl/eval_if.hpp:38: error: no type named ‘type’ in ‘struct boost::mpl::next<boost::mpl::s_iter<boost::mpl::set_c<int, 1l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l, 9223372036854775807l>, boost::mpl::set0_c<int> > >’