The ones I could see

I'm reading The City and the City by China Miéville at the moment. I'm only about 60 pages in but I already really like it, partly for its clever ideas which are currently just emerging, but also for its languages. Miéville has a linguistics background and this shows in the plot of some books (Embassytown, for example) and the writing in all of them: he's creative and clever and does really unusual things with words. Not that you need to have a linguistics background to be able to do that, but it seems like he really knows what he's doing with language. I don't know, I know nothing about literature, so I can only waffle in an uninformed way on that point. But his sentence structure is so clever sometimes, and in this book the characters are from somewhere in central Europe, so the names are all made up but completely believable, and things like that. I think there was even some zeugma at one point and I really love zeugma. 

[Very slight spoilers follow, maybe? Not sure, I haven't read enough of it myself to know.] 

One thing that made me stop and admire it today was a clever use of could. There are parts of the city that the main character can't see. The sentence that caught my eye was describing some railway arches. He says (my bolding):

Not all of them were foreign at their bases. The ones I could see contained little shops and squats decorated in art graffiti.

At this point in the story, the reader doesn't know what the concept is here. Is it that he was able to see the arches, or that he was permitted to see them? This latter idea is what other events have hinted towards, but so far it could be either. 

This is because can and could (present and past tense respectively) can mean two different things. The first meaning, where he was able to see the arches, illustrates logical modality, or the possibility of him seeing them. The second, where he was permitted to see them, illustrates deontic modality: what is possible within the rules. This is an absolutely lovely use of this ambiguity which allows him not to reveal too much of this conceptual device this early in the story. I'm pretty sure he knew exactly what he was doing when he wrote it that way. 

This ambiguity is also, by the way, why you get smartarses responding to a question like 'Can I go to the toilet?' with 'I don't know, can you?'. 

Dogs shall be carried

I was in London recently, and while I was at King's Cross station I noticed that the escalators, which bear the usual safety warnings, now say among other things that 'Dogs shall be carried'. Maybe they've always said that, but it's more common to see the warning as 'Dogs must be carried'. This has given rise to the well-known joke, also seen in the recent Paddington film, in which a child is late for school and when asked why, he says that the sign said dogs must be carried and it took him ages to find a dog.

So my question is, does using 'shall' instead of 'must' remove this comedy interpretation or improve the sign in any way? No one really uses 'shall' any more, but it indicates future and apparently has some sense of being a command. 'Must' also expresses a command ('deontic necessity' - it can also express 'epistemic necessity' as in 'It must be cold outside'). Does 'shall' seem politer? Does a warning sign need to be polite? Unknown. Some more good suggestions here.

(photo credit not known; it's all over the internet)