> If we make working-directory a parameter, how will it work if I want to
> change the directory relative to the current one?

 From SRFI 39: (make-parameter init [converter])

"When it is called with one argument, the content of the cell bound to
this parameter object in the current dynamic environment is set to the
result of the call (converter arg), where arg is the argument passed to
the parameter object"

If we use a converter that turns relative pathnames to absolute ones,
that should solve our problems. I hope the converter can refer to the
old value of the parameter internally.

R7RS supports the converter argument also.

> Gauche's current-directory always returns the absolute pathname, even set
> with relative pathname.

This is good.