Member dedekind::relational::apply (const Comprehension< 𝔸< std::pair< T1, T2 >, L, C >, P > &r, const T1 &x): (documentation-level; #840). apply returns the fibre as a capturing lambda, so its Set carries an opaque, anonymous, non-equality_comparable predicate type: two apply results are never type-equal and the Set cannot round-trip through as_relation. Prefer fibre(R,x) below — it reifies the same fibre as the named FibrePredicate (a stable, inspectable, GraphPredicate / PreimagePredicate-style type). apply is retained (no [[deprecated]] attribute, to keep its existing call sites -Werror-clean) only for the throw-away, reduce-it-now case.