Tony Garnock-Jones wrote:
> a definition for an 
> appropriate zero

Another option is to make zero in Smalltalk inhabit multiple types! (In 
a similar way to the way nil in Smalltalk inhabits multiple types) Then,

sum
   ^ self inject: 0 into: [:accum :each | accum + each]

...would work, so long as the elements treated 0 as a zero for #+.