Linguistics: What are the best packages to use create bracketed structures with movement lines?

I don't know about the best way to create the bracketed structures, but here I show how to insert the so-called movement lines: connect[direction]{left-end}{mid-text}{right-end}. Here is the MWE.

documentclass{article}
usepackage{stackengine,amssymb}
defstacktype{L}
newsaveboxtmpbox
defrlht{3ex}    defrlwd{.8pt} defrloffset{3pt}
newcommandvstrut[1][x]{%
  defstacktype{S}%
  if x#1%
    smash{makebox[0pt]{rule{rlwd}{rlht}}}%
  else%
    smash{makebox[0pt]{stackon[-5pt]{rule{rlwd}{rlht}}%
          {hspace{.5dimexprrlwdrelax}$blacktriangle$}}}%
  fi%
}
newcommandconR[2][x]{%
  saveboxtmpbox{#2}%
  stackunder[dimexprrlht-rlwdrelax+rloffset]{usebox{tmpbox}}{%
    rule{.5wdtmpbox}{0pt}vstrut[#1]rule{.5wdtmpbox}{rlwd}}%
}
newcommandconL[2][x]{%
  saveboxtmpbox{#2}%
  stackunder[dimexprrlht-rlwdrelax+rloffset]{usebox{tmpbox}}{%
    rule{.5wdtmpbox}{rlwd}vstrut[#1]rule{.5wdtmpbox}{0pt}}%
}
newcommandconM[1]{%
  saveboxtmpbox{#1}%
  stackunder[dimexprrlht-rlwdrelax+rloffset]{usebox{tmpbox}}{%
    rule{wdtmpbox}{rlwd}}%
}
newcommandconnect[4][x]{%
  if l#1conR[a]{#2}elseconR{#2}fi%
  conM{#3}%
  if r#1conL[a]{#4}elseconL{#4}fiignorespaces%
}
newcommandlb[1]{[$_mathrm{scriptscriptstyle#1},$}

begin{document}
lb{TP}itshape Seve%
connect[l]{r}{al riotsupshapelb{T}are]lb{VP}lb{V}believed]lb{TP}%
  lb{T}to]lb{AUXP}lb{AUX}have]lb{VP}lb{V}occurred]lb{QP}}{t}%
upshape]]]]]]
end{document}

You know, actually there is an example of such a thing in the end of the gb4e documentation.

documentclass{article}
usepackage{gb4e}
begin{document}
begin{exe}
exatcenter{arrowalign{
lb{TP} & emph{Seveal riots} & lb{T}are]lb{VP}lb{V}believed]lb{TP}lb{T}to]lb{AUXP}lb{AUX}have]lb{VP}lb{V}occurred]lb{QP}&emph{t}&]]]]]] cr
&fillrightpu&link1&fillleftvrule & cr}}
end{exe}
end{document}    

But there is a problem with an alignment: number is between an arrow and an example… There is no such a problem with Steven B. Segletes’s solution. First I've tried to do it with tikz-dependency, but there is the same alignment problem.

Personally, I once used linguex in conjunction with pst-jtree. That gives you all the power you need. Please excuse that I shortened the example somewhat. The jtree command has to be called once in the document before the example for some reason. Also note that the document has to be compiled as .ps first.

documentclass[12pt]{article}
usepackage{linguex}
usepackage[usenames,dvipsnames]{pstricks}
usepackage{pst-xkey}
usepackage{epsfig}
usepackage{pst-jtree}

begin{document}

jtree
endjtree
exi. rnode{a1}{X} [CP $ldots$ [TP $ldots$ rnode{a2}{t} $ldots$]]

ncbar[angleA=270, angleB=270, linestyle=dashed]{<-}{a1}{a2}

end{document}