def subseq(letters, subseq, ordered=True):
    """ Returns whether the letters contains the given subsequence.

    >>> subseq('snake', 'sake')
    True
    >>> subseq('snake', 'sea')
    False
    >>> subseq('snake', 'sea', ordered=False)
    True
    """
    for c in subseq:
        if c not in letters:
            return False
        index = letters.index(c)
        if ordered:
            letters = letters[index+1:]
        else:
            letters = letters[:index] + letters[index+1:]
    return True