Thank you very much for your reply!
I wonder if there's way to express exactly one metaphorical meaning but without using lujvo. For example, in English 'ace in the hole' has special meaning, however I suppose such a rather complex idiom should better not be made into a single word lujvo, and without using tanru as well.
Sorry about keep doubting on this problem. When I tell people around me that I'm learning lojban, and introduce some of its concepts to them, the common reaction is usually 'Isn't it too rigid compared with natural language?' or 'Can artistic literature be created with it, without feeling like some text under rules?' or 'Won't it be so regular that some beauty is lost?'. Actually these are also some questions I want to ask. A list of a thousand cmavo for merely grammatical use seems that the language focuses too much on grammar. (Maybe it's the cost of syntax unambiguity?)
I understand that these questions are raised due to a conventional impression on 'logical language' which are not pondered over deeply and thoroughly. Even though I carefully pick up the most delicate sentence I can find in literature, it's still under the grammar of that language in which the sentence is written. That makes me think maybe grammar doesn't actually constrain semantics as much as I used to feel. But I'm not quite sure about that.
The doubt probably won't be dispelled until I actually see some creative text originally written in lojban to convince me enough. I noticed there's translation for Alice. My current lojban skills still can't support me to read it, so I will keep on learning and come back to see this question.
Yet another question, how are gismu semantics be chosen? i.e. How to determine if a certain meaning is 'basic enough'?
And finally, thanks again for answering my beginner question!