60. Opening disc files
    ~~~~~~~~~~~~~~~~~~
When you open files on a disc for input, using the commands OPENIN or OPENUP,
the syntax is basically chan=OPENIN("myfile"). The first file to be opened is
allocated a channel number like 1, (ie. the variable 'chan' becomes 1), and
if you open another file without closing the first, it is allocated the next
channel number, (eg 2), and so on. The numbers don't necessarily start at 1;
that's just an example.

On most DFSs and ADFS, an interesting thing happens when using OPENIN or
OPENUP, where the filename should already exist on disc, but cannot be found,
either because you have put the wrong disc in, or maybe because you have
mistyped the filename. Instead of causing an error, as you might expect, it
simply allocates channel number zero. Thus, you have a very useful way of
trapping the mistake before it actually causes an error. Eg. you could add
something like:

 IF chan=0 THEN PRINT "Oops!":GOTO 200

There is no need to use CLOSE#, as the file hasn't been opened in the first
place. Only when you attempt to use INPUT#/PRINT#/BGET#/BPUT# on this
non-existent file, does the DFS return the error "Channel", (ERR=222).