public CellSet followCell(Cell cell, Cell blocked) {
if (isIntersection(cell)) return followIntersection(cell, blocked);
if (isCorner(cell)) return followCorner(cell, blocked);
if (isLine(cell)) return followLine(cell, blocked);
if (isStub(cell)) return followStub(cell, blocked);
if (isCrossOnLine(cell)) return followCrossOnLine(cell, blocked);
System.err.println("Umbiguous input at position " + cell + ":");
TextGrid subGrid = getTestingSubGrid(cell);
subGrid.printDebug();
throw new RuntimeException("Cannot follow cell " + cell + ": cannot determine cell type");
}
