@article{ShtrakovKoppitz2016,
  author    = {Slavcho Shtrakov and J{\"o}rg Koppitz},
  title     = {Stable varieties of semigroups and groupoids},
  journal   = {Algebra universalis},
  volume    = {75},
  pages     = {85 -- 106},
  doi       = {10.1007/s00012-015-0359-7},
  year      = {2016},
}