Model: New FilterUnion

[gflima]

Update the FilterPattern class hierarchy to:

• FilterPattern (abstract)
  • BasicFilterPattern or StatementPattern (the current implementation)
  • UnionFilterPattern (union of 2 or more filter patterns)

Implementation of UnionFilterPattern:

1. If no snak sets occur in the child patterns, then each child pattern becomes an entry in the VALUES clause of filter.
2. Otherwise, each child pattern becomes a separate call to filter() and these calls are merged by the union.

[gflima]

See #24

[gflima]

Superseded by the pattern API.

[gflima]

Nothing something urgent. I should move this to TODO.

[gflima]

Moved to TODO.