Create line between two edges in pstree

Question

Basically I want to create a (dashed) line between two tree edges. I tried cheating a bit and created such line manually between two hidden nodes. The problem is, I don't get them hidden! There's always a bit of space between the connecting edges and no parameter I tried seemed to help (radius, treenodesize,..).

\documentclass{minimal}

\usepackage{pstricks,auto-pst-pdf}
\usepackage{pst-tree}

\newcommand{\terminal}[1]{\TR{\boldsymbol{#1}}}
\newcommand{\nonterminal}[1]{\TR{#1}}
\newcommand{\emptyword}{\TR{\epsilon}}
\newcommand{\derivationtree}[1]
{
    \begin{postscript}
        $   \pstree[treemode=R,nodesep=2pt,treesep=0.4cm,levelsep=1.5cm]{\TR{S}}{#1} $
    \end{postscript}    
}

\begin{document}

\derivationtree{
  \pstree{\nonterminal{M_1}}
  {
    \pstree{\Tr[name=m1]{}}
    {
      \terminal{(}
    }
  }
  \pstree[levelsep=1cm]{\nonterminal{S}}
  {
    \emptyword
  }
  \pstree{\nonterminal{M_2}}
  {
    \pstree{\Tr[name=m2]{}}
    {
      \terminal{)}
    }
  }
  \ncline[linestyle=dashed]{m1}{m2}
}

\end{document}

Answer 1

set the nodesep to zero:

\documentclass{minimal}
\usepackage{pstricks,auto-pst-pdf}
\usepackage{pst-tree}

\newcommand{\terminal}[1]{\TR{\boldsymbol{#1}}}
\newcommand{\nonterminal}[1]{\TR{#1}}
\newcommand{\emptyword}{\TR{\epsilon}}
\newenvironment{derivationtree}[1]{%
    \begin{postscript}
        $   \pstree[treemode=R,nodesep=2pt,treesep=0.4cm,levelsep=1.5cm]{\TR{S}}{#1} $
    \end{postscript}}

\begin{document}

\derivationtree{%
  \pstree[nodesep=0]{\nonterminal{M_1}}
  {
    \pstree{\Tr[name=m1]{}}
    {
      \terminal{(}
    }
  }
  \pstree[levelsep=1cm]{\nonterminal{S}}
  {
    \emptyword
  }
  \pstree[nodesep=0]{\nonterminal{M_2}}
  {
    \pstree{\Tr[name=m2]{}}
    {
      \terminal{)}
    }
  }
\ncline[linestyle=dashed,nodesep=0.1pt]{m1}{m2}
}

\end{document}

or as an alternative use the offset option:

\documentclass{minimal}
\usepackage{pstricks,auto-pst-pdf}
\usepackage{pst-tree}

\newcommand{\terminal}[2][]{\TR[#1]{\boldsymbol{#2}}}
\newcommand{\nonterminal}[1]{\TR{#1}}
\newcommand{\emptyword}{\TR{\epsilon}}
\newenvironment{derivationtree}[1]{%
    \begin{postscript}
        $   \pstree[treemode=R,nodesep=2pt,treesep=0.4cm,levelsep=1.5cm]{\TR{S}}{#1} $
    \end{postscript}}

\begin{document}

\derivationtree{%
  \pstree[skiplevels=1,levelsep=4cm]{\nonterminal{M_1}}
    { \terminal[name=m1]{(} }
  \pstree[levelsep=1cm]{\nonterminal{S}}
    { \emptyword }
  \pstree[skiplevels=1,levelsep=4cm]{\nonterminal{M_2}}
    { \terminal[name=m2]{)} }
  \ncline[linestyle=dashed,nodesep=-1.5mm,offset=-2cm]{m1}{m2}
}

\end{document}