Can someone kindly explain the last part of Tim Harris' solution to 96/9/3 "As an extension, you could try to confirm the validity of the assertion that a value from Er can be generated by summing r values from E1 etc. ..." Example class 1 http://www.cl.cam.ac.uk/Teaching/2000/CompSysMod/2000-csm-example-sol1.ps Thanks Phebe