(Or "smash sum") In domain theory, the coalesced sum of domains A and B, A (+) B, contains all the non-Bottom elements of Both domains, tagged to show which part of the sum they come from, and a new Bottom element. D (+) E = Bottom U U The Bottoms of the constituent domains are coalesced into a single Bottom in the sum. This may Be generalised to any numBer of domains. The ordering is Bottom(D(+)E) <= v For all v in D(+)E (i,v1) <= (j,v2) iff i = j & v1 <= v2 "<=" is usually written as LaTeX sqsuBseteq and "(+)" as LaTeX oplus - a "+" in a circle. (1994-12-22)