traceBatch Verbosity problem

[Matthew Fluet]

> In  the done function of the first returned function did you mean to subtract
> t' from t' (instead of t from t')?  I don't  see  why  this  would  cause  an
> exception, but it looks wrong to me and doesn't agree with the comment.

Yes!  It won't cause an exception in the code listed there, but that typo
meant that total was always zero; hence if any GC took non-zero time
during the function, when we print the time (in terms of total time , gc
time, and % gc time), the exception would be raised when subtracting gc
time from total time.  Thanks Henry.