Delayed Processing of Disjunctions

[DavePearce]

At the moment, the splits arising from disjuncts are processed "eagerly". Imagine this is our situation, and we are processing additions in the current state:

+-----------+          +-----------+
| +(x || y) |          | +(x==y+1) |
|           | -------> |           |
| -(1 < 2)  |          |           |
+-----------+          +-----------+
  (current)

Here, we are about to process the disjunct. However, we have already processed some other term which produced a new state with the given delta (+(x==y+1)). Processing the disjunct will cause a split leading to the following (temporary) position:

+-----------+          +-----------+          +-----------+
| +(x || y) |          | +(x==y+1) |          | +(x)      |
|           | -------> |           | ---+---> |           |
| -(1 < 2)  |          |           |    |     | -(x || y) |
+-----------+          +-----------+    |     +-----------+
  (current)                             |
                                        +---> +-----------+
                                              | +(y)      |
                                              |           |
                                              | -(x || y) |
                                              +-----------+

The key is that the next state to be processed will be the first split state (marked next above). At this point, the delta between current and next will be computed as: +(x==y+1), +(x), -(x || y). This is suboptimal because it means the term x == y+1 will be processed separately on both branches.

[DavePearce]

Basically, we need a mechanism for "deferring" facts when they arise. This shouldn't be too hard I guess.