“Extra type variables in representing set” while defining type in Isabelle
问题 I'm working on a theory that uses topology and it would be helpful to have a type of open sets. I tried the following: context topology begin typedef openset = "{U. U ∈ T}" end where topology is a locale and the context command correctly gives the output locale topology = fixes T :: "'a set set" assumes "topology T" However, I get the following error: Extra type variables in representing set: "'a" The error(s) above occurred in typedef "openset" What does it mean? Here T is just a set of sets