  In this paper, we report the line-locking phenomenon of the blended narrow
absorption lines (NALs) within trough-like broad absorption lines (BALs) in
quasar SDSS J021740.96--085447.9 (hereafter J0217--0854). Utilizing the
two-epoch spectroscopic observations of J0217--0854 from the Sloan Digital Sky
Survey, we find that each of its C\,{\footnotesize IV}, Si\,{\footnotesize IV},
N\,{\footnotesize V}, and Ly$\alpha$ BAL troughs actually contain at least
seven NAL systems. By splitting these BAL troughs into multiple NAL systems, we
find that the velocity separations between the NAL systems are similar to their
doublet splittings, with some of them matching perfectly. Cases like
J0217--0854, showing line-locking signatures of NALs within BAL troughs, offer
a direct observational evidence for the idea that radiative forces play a
significant role in driving BAL (at least for Type N BAL) outflows.
