Thanks Peter for the explanation. Nevertheless, I'm not sure that the normal user could have guess that one only gets the clever stuff (compatible embeddings into the algebraci closure. The docstring GF? does imply this but again does not make a big thing of it.
I think a better design (just of the user interface) would be to separate these two things: (i) whether or not you want the field being constructed to be part of the potentially costly lattice of fields, and (ii) whether or not you want to pick your own name. Kwankyu's point is also a good one. It really is not acceptable (from a user's point of view) to ask if there any coercions, be told there are none, and then be prevented from defining one! Meanwhile, now we know what is going on we can work round it (here we is me + my student Marco Caselli, who came to me saying that it appeared to be impossible to iterate over a finite field if it was constructed as an extension of another -- which is true if you construct things in a perfectly innocent way without realising what is happening behind the scenes). John On 13 October 2016 at 18:25, Kwankyu Lee <ekwan...@gmail.com> wrote: > Hi, > > On Thursday, October 13, 2016 at 7:14:27 PM UTC+2, Peter Bruin wrote: >> >> Even though there is no coercion map in this situation, simply trying to >> discover a coercion "taints" the target parent by registering the fact >> that there is no coercion. > > > Then should we consider this behavior as a bug? There is a coercion rule > that there can be at most one coercion from a parent to another parent, but > it should not be disallowed to register one when there is none yet. Right? > > -- > You received this message because you are subscribed to the Google Groups > "sage-devel" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sage-devel+unsubscr...@googlegroups.com. > To post to this group, send email to email@example.com. > Visit this group at https://groups.google.com/group/sage-devel. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To post to this group, send email to firstname.lastname@example.org. Visit this group at https://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/d/optout.