observation
What each observation and each lever is measured in, positionally, null where none was declared (§4.2.4).
A Double in a positional array carries no units, and §4.2.4 states plainly that a wrong-but-plausible declaration — jobs where server-units were meant — is undetectable by anything in this design. These are what let a rule detect it for itself: a rule that knows what it was written for can compare, in configure or at the first epoch, and refuse rather than produce a plausible wrong answer. The library cannot make that check because only the rule knows the answer; what it can do is carry the question.
Declared shape, not epoch-scoped: constant for the life of the element.