Function to_domain_level_default_impl 

pub fn to_domain_level_default_impl<DomL, DecL>(decl_level: DecL) -> DomL
where
    DecL: FuncMap<DeclarationPtr, Moo<Domain>, Output = DomL>,

Implement [ReprDeclLevel::to_domain_level] as a functor S<DeclarationPtr> -> S<DomainPtr>.