anchor.diff:  
   [<Test>]
   let ``exact cover problems with no solutions``() =
  -    let body = fun matrix ->
  +    let body matrix =
           let dlx = new Dlx()
           let solutions = dlx.Solve matrix |> Seq.toList
           solutions.Length = 0 |@ sprintf "Expected no solutions but got %d" solutions.Length
  
positive.code: 
  [<Test>]
  let ``exact cover problems with a single solution``() =
      let body = fun matrix ->
          let dlx = new Dlx()
          let solutions = dlx.Solve matrix |> Seq.toList
          let p1 = solutions.Length = 1 |@ sprintf "Expected exactly one solution but got %d" solutions.Length
  
negative.code:             | 0 -> (numZeros + 1, numOnes)
              | 1 -> (numZeros, numOnes + 1)
              | _ -> (numZeros, numOnes)
          let numZeros, numOnes = Seq.fold innerLoop (0, 0) solution.RowIndexes
          let p1 = numOnes = 1 |@ makeLabel1 colIndex numOnes
          let p2 = numZeros = expectedNumZerosPerColumn |@ makeLabel2 colIndex numZeros
          let p3 = p1 .&. p2
  