import itertools


def fast_forward_to_length(sequences, length):
    """
    Return an itertools.dropwhile that starts from
    the first sequence that has the given length.
    >>> list(fast_forward_to_length([list(range(n)) for n in range(6)], 4))
    [[0, 1, 2, 3], [0, 1, 2, 3, 4]]
    """
    return itertools.dropwhile(lambda seq: len(seq) != length, sequences)