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