docs: def eci2ecef (x, y, z, gmst=None):
    """Converts the given ECI coordinates to ECEF at the given Greenwich
    Mean Sidereal Time (GMST) (defaults to now).
    
    This code was adapted from
    `shashwatak/satellite-js <https://github.com/shashwatak/satellite-js/blob/master/src/coordinate-transforms.js>`_
    and http://ccar.colorado.edu/ASEN5070/handouts/coordsys.doc
  
    """
    if gmst is None:
      gmst = dmc.toGMST()
  
    X = (x * math.cos(gmst))    + (y * math.sin(gmst))
    Y = (x * (-math.sin(gmst))) + (y * math.cos(gmst))
    Z = z
  
    return X, Y, Z