Luabind and passing classes

I was reading through some tutorials and I had a question unanswered.
How do I pass a specific class to Lua?  I have a class that represents a
world in my game engine and I need some way to talk with it from Lua.  I
think LuaBind is the answer but I don't know.  I would never want to
create the class from Lua.  Instead I would want to pass a reference to
the one created on the c++ side to Lua and then call functions inside of
it.  Is this possible with LuaBind?  Basically a simple script would
look like this

-- called when map is loaded by engine --
function Init (worldInstance) 
    -- do stuff --
end

-- called after every frame is rendered --
Update = coroutine.wrap( function (worldInstance, timeSinceLastFrame)
    while true do
	-- do stuff + call coroutine.yield() when waiting for things --
    end
end)
						Nathaniel


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