Boost::units with non SI units

This is driving me nuts. I'm trying to do something simple: Convert between si::meter and US foot.

I can do do:

quantity<si::length> l = 1 * si::meters;

but there doesn't seem to be a US based system, e.g. I cannot find

quantity<si::length> l = 1 * us::foot;

Is that correct? What is the intended use of the US base units?

Edit to clarify: All the US based units are defined, but I can't figure out how to properly use them and the documentation is lacking.

After a lot of staring at the radar_beam_height.cpp example, I came up with the following solution:

#include <boost/units/base_units/us/foot.hpp>
#include <boost/units/io.hpp>
#include <boost/units/quantity.hpp>
#include <boost/units/systems/si/length.hpp>
#include <iostream>

namespace us {
typedef boost::units::make_system<boost::units::us::foot_base_unit>::type system;
typedef boost::units::unit<boost::units::length_dimension, system> length;
static const length foot, feet;
} // namespace us

using namespace boost::units;

int main() {
    quantity<si::length> l0 = 1 * si::meters;
    quantity<si::length> l1{1 * ::us::foot};
    quantity<::us::length> l2 = 1 * ::us::foot;

    std::cout << "l0 = " << l0 << std::endl;
    std::cout << "l1 = " << l1 << std::endl;
    std::cout << "l2 = " << l2 << std::endl;

    return 0;
}

The output is as expected:

l0 = 1 m
l1 = 0.3048 m
l2 = 1 ft

Can someone confirm that this is the intended use?