is "of" a cas operator?

[webmasterpdx]

I was doing some experiments to see what XCas functions were implemented (or not) in the HP Prime's CAS implementation. I tried the user_operator command as in:
user_operator("R",(x,y)->(x*y+x+y),Binary) (works in XCas).

I got an error message:
of(['Error: Bad Argument value'])

My question is that even though I'm sure user_operator isn't implemented in the Prime, which part of what I typed was getting the Bad Argument? Is "of" referring to an operator or something?

Thx
-Donald