>How are we supposed to know what is treated as a variable, and what constitutes an expression? The cases where some arithmetic is involved are obvious, but when is VarType() treating object.property, object.member and object.member.property as an expression, and when as a variable? There must be a general rule to this. I'll simply add it to my Brain.Fox.Map and use it further on.

The key is not to rely on VARTYPE() with an object.property if you are not sure that object.property doesn't exist. Use TYPE() for those cases. The only real case that VARTYPE() with return a "U" without a runtime error for an unknown object.property is:

x=newobject("custom")
? vartype(x.foo) && returns "U"

But in this case above, I would still use TYPE() if I was not certain property foo existed.