def METHOD_NAME(self, deform):
    """Apply a nonlinear deformation field, e.g., MNI2Conform_nonl or
    Conform2MNI_nonl.
    `deform` is a 4D array where the last dimension encodes the coordinates
    corresponding to a particular voxel.
    """
    # deform affine : `from` coords -> `from` voxels
    # deform data   : `from` voxels -> `to` coords
    deform_coords = deform.get_fdata()
    self.ch_pos = coordinates_nonlinear(self.ch_pos, (deform_coords, deform.affine))
    self.headpoints = coordinates_nonlinear(
        self.headpoints, (deform_coords, deform.affine)
    )
    if self.landmarks:
        for k, v in self.landmarks.items():
            self.landmarks[k] = coordinates_nonlinear(
                v, (deform_coords, deform.affine)
            )