Append to the string.

template<
    std::size_t M>
constexpr basic_static_string&
operator+=(
    const basic_static_string< M, CharT, Traits >& s);
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constexpr basic_static_string&
operator+=(
    basic_static_string::value_type ch);
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constexpr basic_static_string&
operator+=(
    basic_static_string::const_pointer s);
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constexpr basic_static_string&
operator+=(
    std::initializer_list< basic_static_string::value_type > ilist);
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template<
    typename T>
constexpr basic_static_string&
operator+=(
    const T& t);
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