Usage: (out-pop [ disp ])
If there has been a push on the output, then close that
file and go back to the previously open file. It is an error
if there has not been a push. See Redirecting Output.
If there is no argument, no further action is taken. Otherwise,
the argument should be #t and the contents of the file
are returned by the function.
Arguments:
disp - Optional - return contents of the file