def forceForwardCoordinates(start, end, strand, length):
    """return forward coordinates.

    If strand is negative, the coordinates in a and b
    will be converted. If they are on the positive
    strand, they will be returned as is.

    Arguments
    ---------
    start : int
        Start coordinate
    end : int
        End coordinate
    strand : string
        Strand of interval. The values of "-", "0", "-1" indicate
        a negative strand.
    length : int
        Length of chromosome.
    """
    if strand in ("-", "0", "-1", 0):
        return length - end, length - start
    else:
        return start, end