code: def kilometers(meters=0, miles=0, feet=0, nautical=0):
      
      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
docs: TODO docs.