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Compare supplied luminances, inspect a two-link focus trace and avoid claiming complete accessibility from isolated checks.

Define a transparent substring rule and trace unmatched-to-cleared results using a complete inert local example.

Preserve repeated requests while separating a lookup table from a unique-membership snapshot.

A constructed poster-and-visit example separates linear association from a causal claim.

A worked dictionary and custom checker expose ambiguous units and missing-value meanings.

A worked join shows how three source rows can become four without any new observation.

A five-record worked audit uses explicit keys, cautious normalisation and a conflict quarantine.

A worked comparison explains why averaging group averages can answer the wrong question.

A worked sensitivity table shows how plausible calculations can answer different questions.

Separate argument parsing, validation, calculation and presentation in one small CLI exercise.