I disagree with nothing you said – I'm very familiar with the relational model. I've used keys in my examples, but nothing prevents the concepts from applying

I disagree with nothing you said – I'm very familiar with the relational model. I've used keys in my examples, but nothing prevents the concepts from applying to the more general cases of tuples and sets. (A set is "dynamic" in that its individual values are not addressable in a manner which is determinable at compile time – you (generally) don't know what will be in the set.) To give separate examples without using keys, you can compare a (integer-indexed) tuple to an array. The former is static-heterogeneous; the latter dynamic-homogeneous. And thankfully, most languages (even mainstream dynamically-typed languages like Python) seem to get the distinction, even though ignoring types or implementation, they are nearly identical.