representing summarize via basic operators

On page 18 of the "New Algebra"

http://www.dcs.warwick.ac.uk/~hugh/TTM/APPXA.pdf

D&D express summarize via basic Tutorial D operators. This construction is not obvious to me (especially that I'm not versatile in Tutorial D syntax). Can anybody please clarify it step by step on a simple example?

Suppose we have an input relation:

{ (x=a,y=1), (x=b,y=1), (x=c,y=1) }

What applications of classic Relational Algebra operations would produce the relation

{ (sum=3) }

? Certainly, infinite relations like "PLUS" (p.5) are allowed:-) Received on Wed Aug 23 2006 - 11:47:07 CDT