\documentclass[a4paper, pdftex, 12pt]{scrartcl}
\usepackage{scrtime}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amsmath, amsfonts, amssymb}
\usepackage{prftree}
\usepackage{stmaryrd}
\usepackage{mathtools}
\begin{document}
\thispagestyle{empty}

\begin{displaymath}
\prftree[l,r]{}{[comp$_{ns}$]}
{
  \prftree[l,r]{}{[comp$_{ns}$]}
  {
    \prftree[l,r]{}{[ass$_{ns}$]}
    {
      -
    }
    {
      (\texttt{m:=a}, \sigma_{\bot,\bot}) \rightarrow \sigma_{48,\bot}
    }
  }
  {
    \prftree[l,r]{}{[ass$_{ns}$]}
    {
      -
    }
    {
      (\texttt{n:=b}, \sigma_{48,\bot}) \rightarrow \sigma_{48,18}
    }
  }
  {
    (\texttt{m:=a; n:=b}, \sigma_{\bot,\bot}) \rightarrow \sigma_{48,18}
  }
}
{
  \prftree[l,r]{}{}
  {
    \dots
  }
  {
    \textbf{[1]}\ (\texttt{LOOP}, \sigma_{48,18}) \rightarrow \sigma_{6,6}
  }
}
{
  (\texttt{m:=a; n:=b; LOOP}, \sigma_{\bot,\bot}) \rightarrow \sigma_{6,6}
}
\end{displaymath}

\end{document}