F&O subtract-dateTimes* should say precedes, not follows

The descriptions of subtract-dateTimes-yielding-yearMonthDuration and
-dayTimeDuration say:

"If the normalized value of $srcval1 follows in time the normalized
value of $srcval2, the returned value is a negative duration."

This should say "precedes" rather than "follows", according to the
example.  They would then be consistent with subtract-dates and
subtract-times as well.

Thanks,
Priscilla