> nil is actually a fine "mysticalObject" and it arises 
> naturally in the 
> implementation of #sum that I would favor:
> 
> Collection>>sum
>      "Answers the sum of the elements in the receiver, nil if empty"
>      | sum |
>      self do:[:value|
>          sum := sum ifNil:[value] ifNotNil:[sum + value].
>      ].
>      ^sum
> 
> > But if you want #sum rather than #sumFromZero: then for the 
> numerical 
> > case it sufficies to do
> > 
> >  mySalarySum := myBigSalaryVector sum + 0
> 
> With the above this becomes:
> 
>    mySalarySum := myBigSalaryVector sum ifNil:[0 euro].
> 
> (assumes unit package with Euro support loaded).
> 
> Cheers,
>    - Andreas

+1 on this, most idiomatic looking solution so far since using ifNil: is a
common idiom for defaulting a value.  It also properly handles collections
which might contain nil's by skipping over the nil while summing without
assuming any ordering by trying to use #first.

Ramon Leon