def test_a_rich(seq, a_region_size, a_ratio):
    """
    Testing the number of As found in just downstream of the PAS.

    seq                 target sequence found downstream of the PAS
    a_region_size       number of bases to look at from the PAS
    a_ratio             A content >= to this ratio will be considered an A-stretch

    >>> test_a_rich("AAAAAGGGGGTTTTTTTTTT", 10, .5)
    True
    >>> test_a_rich("AAAAAGGGGGTTTTTTTTTT", 10, .6)
    False
    """
    found_a_rich = False
    # A-rich sequence downstream
    a_counts = seq[:a_region_size].count("A")
    if a_counts > 0:
        a_content = a_counts / float(a_region_size)
        if a_content >= a_ratio:
            found_a_rich = True
    return found_a_rich