Unary closure variant: Op{}(s) returns S for any S-typed operand. Cardinality has no IsClosedUnderUnary for std::negate<> (negation of a positive natural would need to land outside ℕ); that's the structural witness that -Cardinality is closure-forcing rather than closed. More...

Concept definition

template<typename S, typename Op>
concept dedekind::category::IsClosedUnderUnary =  requires(const S& a) {
  { Op{}(a) } -> std::same_as<S>;
}

Detailed Description

Unary closure variant: Op{}(s) returns S for any S-typed operand. Cardinality has no IsClosedUnderUnary for std::negate<> (negation of a positive natural would need to land outside ℕ); that's the structural witness that -Cardinality is closure-forcing rather than closed.