If the value to print is not #!unspecified, the procedure
should print it on the current output port. The default printer is
write in r5rs.heap and pretty-print in larceny.heap
and twobit.heap.
existence of repl
repl behavior in general, notably
- its control of current input and output and how you can't interactively
change current input and output.
- its control of the error handler, reset handler, quit handler, and
sundry interrupt handlers