[LudiusMaximus]

All right, thanks for the clarification! Now I understand the difference between SetScript and HookScript. :-D

However, if other Addons use the same method as you suggested, I have again no controll over which one is loaded first, right?

E.g. the author of LibExtraTip (used by TheUndermineJournal) does it like this (LibExtraTip.lua, L545):

-- Called to install/modify a pre-hook on the given tooltip's event
-- Currently we do not need any posthooks on scripts
local function hookscript(tip, script, prehook)
    if not lib.hookStore[tip] then lib.hookStore[tip] = {} end
    local control
    -- check for existing hook
    control = lib.hookStore[tip][script]
    if control then
        control[1] = prehook or control[1]
        return
    end
    -- prepare upvalues
    local orig = tip:GetScript(script)
    control = {prehook}
    lib.hookStore[tip][script] = control
    -- install hook stub
    local stub = function(...)
        local h
        -- prehook
        h = control[1]
        if h then h(...) end
        -- original hook
        if orig then orig(...) end
    end
    tip:SetScript(script, stub)
end

With your code, my lines always get appended after those of TheUndermineJournal.