But if you model a set in all relation isn't one outside,
the set or the relation, in the theory of either?
Set relation theory.
Function theory and set theory are two different things,
i.e. what in set theory "is a Cartesian model of a function
according to the existence and there are usual models of
at least indicator presence in all the discrete Cartesian,
so that existence proofs are so easy in terms of functions",
here is for variously theory: set theory, part theory, function
theory, type theory, category theory, ....
When you write "f subset domain cross co-domain", even those
don't define all function besides that you made for "defined" in
function.
Of course this is where a special function the line-drawing between
zero and one rests right between set theory and function theory,
though a function not having necessarily a Cartesian model.
(In set theory.)
For whatever the strength of a set theory is and whatever strength
is, there is an object that is a model of each, under the "equi-interpretable",
in terms of the theories being and having a model, this is a ubiquitous
theory with various primary elements like set theory's, category theory,
relations and other theories in terms, and besides what's categorical as
geometry, though that geometry definitely has the continuous, which in
terms is not so much a usual matter of words, plural.
Existence proofs are often by allusion. (And exclusion.)
Then the point of theory and statement is the strength,
here the point is that DC proof is arbitrarily weak, according
to that following adherence admits the validity of inference,
when in reality it is for example a case of slippery-slope.
The "slippery slope" is an example from logic, it's that the
approach makes back-pedaling, or otherwise admits a reason
to admit a reason to admit a reason that under terms makes
sense as an approach, but doesn't work as an approach.
(To reaching soundness, for decision, and in validities.)