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@Book{            book:610591,
  Author        = "Velleman, Daniel J.",
  date-added    = "2020-04-10 11:07:26 +0200",
  date-modified = "2020-04-10 11:07:26 +0200",
  Edition       = "2",
  ISBN          = "0521675995,9780521675994",
  Publisher     = "Cambridge University Press",
  Title         = "How to Prove It: A Structured Approach",
  URL           = "http://gen.lib.rus.ec/book/index.php?md5=477606133efef5b01b3891bb8d7540be",
  Year          = "2006",
  bdsk-url-1    = "http://gen.lib.rus.ec/book/index.php?md5=477606133efef5b01b3891bb8d7540be",
  File          = "How to Prove It - A Structured Approach - a - a - a.pdf",
  file-2        = "How to Prove It - A Structured Approach - b - a - a.pdf"
}

@Book{ book:610591, Author = "Velleman, Daniel J.", date-added = "2020-04-10 11:07:26 +0200", date-modified = "2020-04-10 11:07:26 +0200", Edition = "2", ISBN = "0521675995,9780521675994", Publisher = "Cambridge University Press", Title = "How to Prove It: A Structured Approach", URL = "http://gen.lib.rus.ec/book/index.php?md5=477606133efef5b01b3891bb8d7540be", Year = "2006", bdsk-url-1 = "http://gen.lib.rus.ec/book/index.php?md5=477606133efef5b01b3891bb8d7540be", File = "How to Prove It - A Structured Approach - a - a - a.pdf", file-2 = "How to Prove It - A Structured Approach - b - a - a.pdf" }