I realize `mgcv` does this kind of helpful (or "helpful") fixing-up of the knot argument, but what I'm wondering is if there's any reason to follow suit :-). The compatibility argument doesn't seem very relevant here, since anyone who is transferring results to or from mgcv will have to explicitly specify the knots anyway, right? And if we do error out here, then it's pretty trivial for people to fix -- they just have to explicitly say how many knots mgcv chose. (Also mgcv's behaviour in this case is just weird -- if it's less than the minimum it gets clamped to minimum, unless it's even smaller than that in which case it gets set to the default? wtf?)

(Also: note that if you go to the "Files changed" tab on the pull request, then you can reply to individual line comments directly -- this is helpful for keeping track of discussions like this :-). Replying directly to an email notification will also work.)