def test_for_a_stretch(seq, a_stretch):
    """
    Test whether or not the immediate downstream bases of the PAS are A
    throughout the a_stretch distance.

    >>> test_for_a_stretch("AAAAATTTTTTTTTT", 5)
    'a'
    >>> test_for_a_stretch("AAAATTTTTTTTTTT", 5)
    ''
    """
    return 'a' if seq[:a_stretch].count("A") == a_stretch else ''