calc: whats's the meaning of '{}'

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sorry, still not oriented:

is it an 'empty array'?

saying something like 'no value for an undefined cell'?

only appearence of "{}":

"Constraints: N != {}; Evaluators may evaluate expressions that do not meet this constraint."

to understand that i'd have to dive deep into the context of that statement ...

it's just that special case "{}" - two curly brackets with 'nothing inside' what i'm curious about, {1;2;3} makes sense, {} not - not yet ...

It is an empty inline array - don't ask me why it is used in the formula (I can't grasp the idea behind using "{}" instead of "" either)

"Constraints: N != {}; Evaluators may evaluate expressions that do not meet this constraint."

If you refer to some statements, please cite the place where you find that. The statement is about the fact that the Standard defines empty inline arrays are not allowed (defined) by the standard (Constraint), but Evaluators (i.e. software product implementing the standard) are "allowed" by the standard to implement (and thus circumvent the constraint) their own solution for arrays not meeting the non-empty-requirement.

@Opaque: thanks for clarifying and high respect for your fundamental knowledge ...