What does the question mark mean in Power Query?

[ExcelStarter]

https://community.powerbi.com/t5/Po...ive-numbers-is-it-possible/m-p/1441336#M45058

I saw something weird in smauro's answer.

=List.Accumulate(List.Sort(#"Changed Type"

), {},
  (s, c) =>if ((c - List.Last(s){0}[COLOR=#ff0000]?)??[/COLOR]1) <= 1
            then List.RemoveLastN(s, 1) & {{c, ((List.Last(s){1}[COLOR=#ff0000]?)??[/COLOR]0) + 1}}
        else s & {{c, 1}})

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What does the question mark mean in Power Query?

 

[ExcelStarter]

I found the answer.

https://bengribaudo.com/blog/2020/01/06/4844/power-query-m-primer-part14-control-structure