def trim_left(alleles):
    """Removes common prefix of given alleles.

    Args:
        alleles (list of str): A list of alleles.
        
    Returns:
        tuple: ``(number_trimmed, [new_alleles])``.

    Examples:
        >>> trim_left(["","AA"])
        (0, ['', 'AA'])

        >>> trim_left(["A","AA"])
        (1, ['', 'A'])

        >>> trim_left(["AT","AA"])
        (1, ['T', 'A'])

        >>> trim_left(["AA","AA"])
        (2, ['', ''])

        >>> trim_left(["CAG","CG"])
        (1, ['AG', 'G'])
    """

    trimmed = 0
    while all(len(a) > 0 for a in alleles):
        a0 = alleles[0]
        for a in alleles[1:]:
            if a0[0] != a[0]:
                return trimmed, alleles
        alleles = [a[1:] for a in alleles]
        trimmed += 1
    return (trimmed, alleles)