@article{Klarner1982,
  author    = {{David A. Klarner}},
  title     = {{A sufficient condition for certain semigroups to be free}},
  journal   = {{Journal of Algebra}},
  year      = {{1982}},
  volume    = {{74}},
  pages     = {{140--148}},
  note      = {{MR 83e:10081}},
}