1) SRFI 170 `working-directory` and `umask` are per-thread if and only if the other R7RS procedures that accept pathnames are implemented using per-thread versions in a particular Scheme implementation providing SRFI 170.

2) SRFI 170 does not specify whether these procedures are parameters or not.

3) R7RS does not specify (or prevent) assignment to a parameter as opposed to binding it; therefore `set-working-directory` and `set-umask` are the only portable ways to change their values, whether per-thread or per-process.

4) For the avoidance of doubt, all this will be spelled out in the SRFI text.

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