hi,

May I introduce a suggestion to help the program runtime to check the type of a variable and not crash.

There is already a function DESCRINFO(1,variable) which return the type of the variable ; so some type c.f. descrinfo() doc ref 

on sellist, pyobject, udo, do, xdom, ... descrinfo() return -1 ... 

I'll suggest to implement more descrinfo result value like 11=sellist, 12=pyobject, 13=udo, 14=do, 15=xdom, ...

and by extension, if request_code = 2,  descrinfo return a DO with polymorph properties aka. metadata about the variable. 

with kind regards