It sometimes happens that a user changes their mind about having typed one or more lines of input and would like to `untype' them before the server actually gets around to processing them. If they react quickly enough, they can type their connection's defined flush command; when the server first reads that command from the network, it immediately and completely flushes any as-yet unprocessed input from that user, printing a message to the user describing just which lines of input were discarded, if any.
Fine point: The flush command is handled very early in the server's processing of a line of input, before the line is entered into the task queue for the connection and well before it is parsed into words like other commands. For this reason, it must be typed exactly as it was defined, alone on the line, without quotation marks, and without any spaces before or after it.
When a connection is first accepted by the server, it is given an initial flush
command setting taken from the current default. This initial setting can be
changed later using the set_connection_option() command.
By default, each connection is initially given `.flush' as its flush
command. If the property $server_options.default_flush_command exists,
then its value overrides this default. If
$server_options.default_flush_command is a non-empty string, then that
string is the flush command for all new connections; otherwise, new connections
are initially given no flush command at all.