[ begin{array}{rll}
1&plthen q\
2&qlthen r\
3&p\
4&q & mathrm{E}{lthen} 1,3\
5&r & mathrm{E}{lthen} 2,4\
6&plthen r & mathrm{I}{lthen} 3{-}5\
7&(qlthen r)lthen (plthen r) & mathrm{I}{lthen} 2{-}6
end{array} ]

documentclass{article}
usepackage{amssymb}
letlthento
usepackage{tikz}
usetikzlibrary{tikzmark,calc}
newcounter{lconnect}
begin{document}
[ begin{array}{rll}
tikzmarknode{1}{1}&plthen q\
tikzmarknode{2}{2}&qlthen r\
tikzmarknode{3}{3}&p\
tikzmarknode{4}{4}&q & mathrm{E}{lthen} 1,3\
tikzmarknode{5}{5}&r & mathrm{E}{lthen} 2,4\
tikzmarknode{6}{6}&plthen r & mathrm{I}{lthen} 3{-}5\
tikzmarknode{7}{7}&(qlthen r)lthen (plthen r) & mathrm{I}{lthen} 2{-}6
end{array} ]
begin{tikzpicture}[overlay,remember picture,lconnect/.style args={#1 and #2}{%
/utils/exec=stepcounter{lconnect},insert path={(#1.west)--
++(-1*pgfkeysvalueof{/tikz/lconnect/offset}-1*numbervalue{lconnect}*pgfkeysvalueof{/tikz/lconnect/step},0) |- (#2.west)}},lconnect/.cd,
offset/.initial=0.1em,step/.initial=0.4em]
 draw[lconnect/.list={3 and 5,2 and 6}];
end{tikzpicture}
end{document}