be missing close parentheses for car and cdr.

But I feel that it's better to have timespec as an implementation-dependent

data structure, rather than a pair of integers.  If an implementation want to use a pair, that's ok.

However, an implementation may want to use its own timespec struct to integrate

into the rest of the system (e.g. SRFI-19 time)

We already have timespec-difference and timespec=?, which are technically unnecessary

we do specify timespec is just a pair.   To add (make-timespec seconds nanoseconds),

(timespec? timespec),  (timespec-second timespec) and (timespec-nanosecond timespec),

we can leave the concrete data structure of timespec to implementation's choice.