E.4.CM:5.1 - Assemble a comparison that another reader can use

A team has two public operations: grouping records and comparing paired observations. Its receiving question requires each error/non-error pair to share both setup and temperature. The existing grouping uses only setup.

The available data are T1=(error, S1, warm), T2=(acceptable, S1, cool), T3=(error, S2, warm), T4=(acceptable, S2, warm). Grouping by setup offers T1–T2 and T3–T4. Only the latter satisfies the comparison’s precondition.

The author locates a composition difficulty: the producer’s grouping key is weaker than the receiver’s eligibility rule. The repair groups by setup and temperature, returns T3–T4 as eligible and makes the missing S1 comparison explicit. Comparison then uses the eligible pair; it cannot claim a result for S1. A request for another record is warranted only if that unresolved use matters.

This is already developed in C.40.CD:5.1. E.4.CM therefore selects reuse of that explanation rather than another pattern for these same operations. If a proposed card merely lists grouping and comparison, it should point to the available construction instead of asking the reader to invent the missing key.

Now change the receiving rule to require a shared instrument revision too. Temperature matching alone no longer answers eligibility. If revisions are present, extend the key; if absent, return the unavailable comparison rather than treating omission as a match. This change illustrates what the public explanation must teach: derive the joining rule from the receiving condition and preserve partial results. It does not establish causality from one matched pair.