:</span><span class="identifier">basic_string</span><span class="special">&lt;</span><span class="identifier">char_type</span><span class="special">&gt;</span></code>)
        or be a unary, binary or ternary functor that computes the replacement string
        from a function call: either <code class="computeroutput"><span class="identifier">fmt</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span></code>
        which must return a container of <code class="c