difftime arithmetic (PR#2345)

"MM" == Martin Maechler <maechler@stat.math.ethz.ch>
    on Wed, 4 Dec 2002 18:09:08 +0100 (MET) writes:

MM> We will have to fix that in the C code, too.  Martin

well, probably, not.

I found that we are simply missing an  Ops.difftime()
{method implementation for `arithmetic' generics}.

Martin