e given channel, flushing all buffered write operations.
   Output functions raise a <code class="code"><span class="constructor">Sys_error</span></code> exception when they are
   applied to a closed output channel, except <code class="code">close_out</code> and <code class="code">flush</code>,
   which do nothing when applied to an already closed channel.
   Note that <code class="code">close_out</code> may raise <code class="code"><span class="constructor">Sys_error</span></code> if the operating
   syst