 is passed as argument to <span class="emphasis"><em>coroutine-function</em></span>. The <span class="emphasis"><em>coroutine-function</em></span>
        must call this <span class="emphasis"><em>asymmetric_coroutine&lt;&gt;::push_type::operator()</em></span>
        in order to transfer each data value back to the main-context. In the main-context,
        the <span class="emphasis"><em>asymmetric_coroutine&lt;&gt;::pull_type::operator bool</em></span>
        determines whether the coroutine is still val