A value of type TIO represents the state of the terminal I/O
system. The bol
flag keeps track of whether we are at the
beginning of line on the console. This is computed in terms of
what we have sent to the console, but it should be remembered that
the order that stdout and stderr are sent to the console may not be
the same as the order in which they show up there. However, in
practice this seems to work as one would hope.