def METHOD_NAME(self, reference_coords, coords):
    """Set the coordinates to be superimposed.
    coords will be put on top of reference_coords.
    - reference_coords: an NxDIM array
    - coords: an NxDIM array
    DIM is the dimension of the points, N is the number
    of points to be superimposed.
    """
    # clear everything from previous runs
    self._clear()
    # store coordinates
    self.reference_coords = reference_coords
    self.coords = coords
    n = reference_coords.shape
    m = coords.shape
    if n != m or n[1] != 3 or m[1] != 3:
        raise Exception("Coordinate number/dimension mismatch.")
    self.n = n[0]