Problem with Union Types?

[DavePearce]

The following fails to verify for reasons unknown:

type PieceKind is (int x) where (0 <= x) && (x <= 5)
type Piece is ({bool colour, Complex_Valid_1.PieceKind kind} this)
type Square is (null|Complex_Valid_1.Piece this)
type RowCol is (int x) where (x >= 0) && (x < 8)
type Pos is ({Complex_Valid_1.RowCol col, Complex_Valid_1.RowCol row} this)
type Row is (Complex_Valid_1.Square[] squares) where |squares| == 4
type Board is ({Complex_Valid_1.Row[] rows} $) where |$.rows| == 4

assert "type invariant not satisfied":
    if:
        true
    then:
        {rows: [
            [{colour: false, kind: 0}, {colour: false, kind: 0}, {colour: false, kind: 0}, {colour: false, kind: 0}],
            [{colour: false, kind: 0}, {colour: false, kind: 0}, {colour: false, kind: 0}, {colour: false, kind: 0}],
            [{colour: false, kind: 0}, {colour: false, kind: 0}, {colour: false, kind: 0}, {colour: false, kind: 0}],
            [{colour: false, kind: 0}, {colour: false, kind: 0}, {colour: false, kind: 0}, {colour: false, kind: 0}]
        ]} is Complex_Valid_1.Board

The strange thing about this one is that it has the hallmarks of being related to the verification limit. That is, if we reduce the required dimensions of the board appropriately, say to 2x2, then it works fine. However, adjusting the limit parameter doesn't seem to make any difference ... ?

[DavePearce]

This does verify ... it just needs 5000 steps. The proof is pretty ugly, to say the least.