(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.