Bug or quirk ("expected behaviour")?

tell application id "DNtp"
	-- this gets me an error:
	set listRTF to children of current group whose type is in {rtf, rtfd}
	-- I have to do this:
	set listRTF to (children of current group whose type is rtf) & (children of current group whose type is rtfd)
end tell

```I have often used the first form in other applications, but DT does not allow it.
Is it, maybe, just an omission in DT's understanding of A/S?

Why would it be “a misunderstanding”? There is no “standard” on this, only implementations. Just because it worked in one (or some other applications), that does not mean it should (or must) work in others.

For something a little less implementation-specific, and a bit faster anyway (booleans are built into the subsyntax of where/whose clauses)

tell application id "DNtp"
    set listRTF to (children of current group where its type is rtf or its type is rtfd)

end tell

Thank you, gentlemen, for showing me that I was being led by perceived patterns. I have to work on that…

No worries. It would actually be nice if there was a standard on some things like this, but it’s more akin to dialectical differences in spoken language. I can go to the Southeast part of the US and understand what people say, but they will pronounce things a bit differently than Northerners and will have some phrases I may not understand. However, we are both speaking American English. :smiley: