TODO docs.

def kilometers(meters=0, miles=0, feet=0, nautical=0):
    """
    TODO docs.
    """
    ret = 0.
    if meters:
        ret += meters / 1000.
    if feet:
        ret += feet / ft(1.)
    if nautical:
        ret += nautical / nm(1.)
    ret += miles * 1.609344
    return ret