function xkbion_set (groups) -- the only global created by xkbion.lua

	if not groups or type(groups) ~= "table" then
		error("bad args")
	end

	if not groups[1] or type(groups[1]) ~= "table" then
		error("default group is undefined")
	end

	local window_group_prop
	do
		local XA_INTEGER = 19
		local atom = notioncore.x_intern_atom( tostring( groups.atomname or "XKBION_GROUP" ) )
		if not atom or type(atom) ~= "number" then
			error("Cannot intern atom " .. atomname)
		end
		window_group_prop = function(w, gnum)
			if not w or type(w) ~= "userdata" or not w.xid or type(w.xid) ~= "function" then return 1 end
			local xid = tonumber( w:xid() )
			if gnum == nil then
				local t = notioncore.x_get_window_property( xid, atom, XA_INTEGER, 1, true )
				if t and type(t) == "table" and t[1] ~= nil then
					do return tonumber(t[1]) end
				else
					gnum = 1
				end
			else
				gnum = tonumber(gnum)
			end
			-- we're here if the second argument is set or if the window does not have our property yet
			notioncore.defer( function()
				notioncore.x_change_property( xid, atom, XA_INTEGER, 32, "replace", {gnum} )
			end )
			return gnum
		end
	end

	local set_group
	do
		local current_gnum = 1
		local first_time = true
		local statusname = groups.statusname

		if statusname and type(statusname) ~= "string" then
			statusname = nil
		end

		set_group = function(w, do_increment)
			local gnum
			if w then
				gnum = window_group_prop(w)
			else
				gnum = 1
			end
			if do_increment then gnum = gnum + 1 end
			local g = groups[gnum]
			if not g then gnum, g = 1, groups[1] end
			if not g then return end -- error in settings, groups[1] not defined
			if first_time then
				first_time = false
			elseif gnum == current_gnum then
				return
			end
			window_group_prop(w, gnum) -- it's OK to call it even it `w' is nil
			if g.command then
				notioncore.exec(g.command)
			end
			if g.action then notioncore.defer(g.action) end
			current_gnum = gnum
			local group_name = g.name
			local hint_name = g.hint
			if statusname and group_name and type(group_name) == "string" then
				mod_statusbar.inform(statusname, group_name)
				mod_statusbar.inform(statusname.."_hint", hint_name)
				notioncore.defer(mod_statusbar.update)
			end
		end
	end

	notioncore.get_hook("region_notify_hook"):add(
		function(reg, action)
			if (reg ~= nil) and (tostring(reg.__typename) == "WClientWin") then
				if (action == "activated") or (action == "pseudoactivated") then
					set_group(reg)
				end
			end
		end
	)

	local key = groups.key
	if key and type(key) == "string" then
		defbindings("WClientWin", {
			kpress(key, function (_, _sub) set_group(_, true)  end)
		})
	end

	set_group() -- initialize
end