[Dawn]

To make life a little easier, I'm using this

-- set/clear focus with shift + left click
    local ModKey = 'Shift'
    local MouseButton = 1
    local key = ModKey .. '-type' .. (MouseButton or '')
	if(self.unit == 'focus') then
		self:SetAttribute(key, 'macro')
		self:SetAttribute('macrotext', '/clearfocus')
	else
		self:SetAttribute(key, 'focus')
	end

and this

-- remove SET_FOCUS & CLEAR_FOCUS from menu, to prevent errors
do 
    for k,v in pairs(UnitPopupMenus) do
        for x,y in pairs(UnitPopupMenus[k]) do
            if y == "SET_FOCUS" then
                table.remove(UnitPopupMenus[k],x)
            elseif y == "CLEAR_FOCUS" then
                table.remove(UnitPopupMenus[k],x)
            end
        end
    end
end