@article{SimpsonStreicher:12,
  author = {Simpson, Alex and Streicher, Thomas},
  pages = {1419-1436},
  title = {Constructive toposes with countable sums as models of constructive
           set theory.},
  volume = {163},
  year = {2012},
  number = {2},
  journal = {Annals of Pure and Applied Logic},
}