def get_start_end(sequence, skiplist=('-', '?')):
    """Return position of first and last character which is not in skiplist.

    Skiplist defaults to ['-','?'].
    """
    length = len(sequence)
    if length == 0:
        return None, None
    end = length - 1
    while end >= 0 and (sequence[end] in skiplist):
        end -= 1
    start = 0
    while start < length and (sequence[start] in skiplist):
        start += 1
    if start == length and end == -1:  # empty sequence
        return -1, -1
    else:
        return start, end