Overload set ref

template<class CharSet>
constexpr
enable_if<is_charset<CharSet>::value && !std::is_same<CharSet, detail::charset_ref<CharSet>>::value, charset_ref<CharSet>>::type
ref(const CharSet& cs) noexcept;
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template<class Rule>
constexpr
enable_if<is_rule<Rule>::value && !std::is_same<Rule, detail::rule_ref<Rule>>::value, rule_ref<Rule>>::type
ref(const Rule& r) noexcept;
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constexpr
void
ref() = delete;
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