tbl_cell_span
[0|1] = tbl_cell_span(upperLeftCellId, lowerRightCellId)
This function combines the rectangle of cells specified by upperLeftCellId and lowerRightCellId into a multicell.
It does this by constructing a new rectangle with the coordinates [sumUpperLeft, sumLowerRight]
where:
sumUpperLeft.X = min(oldUpperLeft.X, newUpperLeft.X)
sumUpperLeft.Y = min(oldUpperLeft.Y, newUpperLeft.Y)
sumLowerRight.X = max(oldLowerRight.X, newLowerRight.X)
sumLowerRigth.Y = max(oldLowerRight.Y, newLowerRight.Y)
and trying to span that.
This will fail if any cell in the resulting rectangle is a member of any other multicell.