--- Any instance must define either 'arr' or 'pure' (which are synonyms),
--- as well as 'first'. The other combinators have sensible
--- default definitions, which may be overridden for efficiency.
+--
+-- Minimal complete definition: 'arr' and 'first'.
+--
+-- The other combinators have sensible default definitions,
+-- which may be overridden for efficiency.