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I can't seem to figure out what's wrong with this.
\documentclass[12pt]{report}
\usepackage{tikz-qtree}

\begin{document}
\begin{tikzpicture}[level distance=70pt,sibling distance=50pt]
\Tree [.$(x<0)$
\edge node[auto=right]{1};
[.(a)
\edge node[auto=right]{1};
[.$f_1();$ ]
\edge node ...