Performs the same actions as if the connection <conn>'s definied flush command had been received on that connection, i.e., removes all pending lines of input from <conn>'s queue and, if <show-messages> is provided and true, prints a messages to <conn> listing the flushed lines, if any. See the chapter in the LambdaMOO Programmer's Manual on server assumptions about the database for more information about a connection's defined flush command.