What does the question mark mean in Power Query?

[ExcelStarter]

https://community.powerbi.com/t5/Pow...1441336#M45058

I saw something weird in smauro's answer.

=List.Accumulate(List.Sort(#"Changed Type"[Code]), {},
(s, c) =>if ((c - List.Last(s){0}?)??1) <= 1
then List.RemoveLastN(s, 1) & {{c, ((List.Last(s){1}?)??0) + 1}}
else s & {{c, 1}})

====================================================

What does the question mark mean in Power Query?

[ExcelStarter]

I found the answer.

https://bengribaudo.com/blog/2020/01...trol-structure