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tokens: an injection of excess twist into the filament from one footpoint could drive a combination of longitudinal and rotational plasma motions (Uchida et al. 2001 ; Vršnak et al. 2007 ). However, when the Alfvén-wave packet caused by the injection is reflected at the opposite footpoint, the direction of rotational motions is expected to reverse, but in our case, the Doppler pattern persists (Figure 4 ). Moreover, we found no sign of twisting motions in the surge. Hence, we dismissed the wave-packet interpretation for the current observation. But we cannot dismiss the possibility that the observed rotational motion is a manifestation of unwinding relaxation; in other words, magnetic twist is propagating out of rather than into the filament, which could happen when the twisted filament field reconnects with the ambient, untwisted field (Okamoto et al. 2016 ). Additionally, Jing et al. ( 2003 , 2006 ) speculated that a mass perturbation in a fine thread would propagate along the spine across the obliquely stacked threads making up the filament. In our high-resolution observations, it is clear that mass motions involved in longitudinal oscillations are directed along individual threads, but it is obscure whether there are any perturbations propagating across the threads. To conclude, this study has demonstrated a promising methodology to diagnose the magnetic configuration of filaments and has substantiated the presence of the double-decker configuration with a flux rope atop a sheared arcade (Liu et al. 2012 ). Since no discernible separation is seen inside the filament studied, such a configuration might be more common than previously thought. With this configuration, however, one expects normal polarity in the lower atmosphere but inverse polarity in the higher atmosphere; because of the mixed signs of helicity, one also expects two pairs of plasma draining sites if the filament fully erupts, except that double-decker filaments often erupt partially (e.g., Liu et al. 2012 ; Cheng et al. 2014 ; Zhu et al. 2015 ). These predictions remain to be verified by future observation. Appendix Extraction of Oscillatory Patterns From the representative TD maps selected along the spine (Figure 6 ), we extracted the oscillatory pattern through a semi-automatic procedure that starts by visually choosing 20 points outlining the entire discernible pattern (Figure 7 (b)). Next, we applied a cubic spline to interpolate the intermediate points (Figure 7 (b)). We further refined the interpolations if the intensity profiles deduced from the TD maps could be well fitted by a Gaussian function ( gaussfit.pro ; Figure 7 (c)). Each intensity profile is 20 pixels wide, centering on the minimum intensity location within ±15 pixels of the original interpolated location. The full-width-half-maximum of the best-fit Gaussian function is considered as the uncertainty of the oscillation amplitude. We then fitted the extracted oscillation pattern by the exponentially