@Article{         Langford:AM:1926:B,
  Author        = "Langford, C. H.",
  date-added    = "2020-05-31 11:27:08 +0200",
  date-modified = "2020-05-31 11:27:51 +0200",
  ISSN          = "0003486X",
  Journal       = "Annals of Mathematics",
  Number        = "1/4",
  Pages         = "459--471",
  Publisher     = "Annals of Mathematics",
  Title         = "Theorems on Deducibility",
  URL           = "http://www.jstor.org/stable/1968390",
  Volume        = "28",
  Year          = "1926",
  bdsk-url-1    = "http://www.jstor.org/stable/1968390",
  File          = "Theorems on Deducibility - langford1926 - a - x.pdf"
}

@Article{ Langford:AM:1926:B, Author = "Langford, C. H.", date-added = "2020-05-31 11:27:08 +0200", date-modified = "2020-05-31 11:27:51 +0200", ISSN = "0003486X", Journal = "Annals of Mathematics", Number = "1/4", Pages = "459--471", Publisher = "Annals of Mathematics", Title = "Theorems on Deducibility", URL = "http://www.jstor.org/stable/1968390", Volume = "28", Year = "1926", bdsk-url-1 = "http://www.jstor.org/stable/1968390", File = "Theorems on Deducibility - langford1926 - a - x.pdf" }