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coalesced sum

<theory> (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(D(+)E) }
U { (0,d) | d in D, d /= bottom(D) }
U { (1,e) | e in E, e /= bottom(E) }
```

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)

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