from typing import List


def isRectangular(L: List[List[int]]) -> bool:
    """Check if the length of each row is equal to the length of the first row."""
    return all(len(row) == len(L[0]) for row in L)


def testIsRectangular():
    print("Testing isRectangular()...", end="")
    L = [[1, 2, 3], [4, 5, 6]]
    assert isRectangular(L) == True
    L = [[1, 2], [4, 5, 6]]
    assert isRectangular(L) == False
    L = [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]
    assert isRectangular(L) == True
    print("Passed!")


def main():
    testIsRectangular()


main()