Use Column header in calculated column

Thank you for the sample data.  Note that it has a lot of extra spaces so the following code may need some adjustments once you cleaned the data up.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8k0sLkktUtJRMgRiR0cQwwDEVFCK1UGSNQJiJyewuI5ScKAPqqwxVMLQAKQuIL88tUjByROsJji/tCg5FShqAlcDUh0c7BmMKm8KxM7OUEVotpsBsYsLWLMJWLMjUHMsAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Filename = _t, #"   Col A" = _t, #"   Col B" = _t, #"   Col C" = _t, #"  Col D" = _t]),
    CN =Table.ColumnNames(Source),
    #"Added Custom" = Table.AddColumn(Source, "Output Col",
  (k)=> let l =
  List.Generate(()=>[x=1,y=""], 
      each [x] < List.Count(CN), 
      each [x=[x]+1, y=[y] & (if Record.Field(k,CN{[x]}) > " " then "" else ", " & CN{[x]})],
      each [y] & (if Record.Field(k,CN{[x]}) > " " then "" else ", " & CN{[x]})
  ),
  m = try Record.Field(k,CN{0}) & "/" & Text.Range(l{List.Count(CN)-2},2) otherwise null
  in m
)
in
    #"Added Custom"

How to use this code: Create a new Blank Query. Click on "Advanced Editor". Replace the code in the window with the code provided here. Click "Done".