How to do y-split for tree nodes in pstree?

Question

As I need to draw lines between edges, I'd like to have y-splits when there are multiple child nodes. At the moment I realized it through a hidden node and nodesep=0. Unfortunately, I don't know of any way to reset nodesep for the direct parent and children of the split node and therefore the spacings are wrong (see screenshot).

\documentclass{minimal}

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

\newcommand{\terminal}[2][]{\TR[#1]{\boldsymbol{#2}}}
\newcommand{\nonterminal}[2][]{\TR[#1]{#2}}
\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[nodesep=0]{\nonterminal{N_1}}
  {
    \pstree{\Tr{}}
    {
        \pstree[nodesep=2pt]{\nonterminal{N_1}}
        {
            \terminal{[}
        }
        \terminal{[}
    }
  }
}

\end{document}

In the pst-tree manual, there is an example where \psedge is redefined as \ncdiag. But I couldn't really figure out how to do it and how to restrict the effects just for one case.

Answer 1

Usually one redefines \psedge but it works with any other definition.

\documentclass{minimal}
\usepackage{pstricks,auto-pst-pdf}
\usepackage{pst-tree}
\newcommand\terminal[2][]{\TR[#1]{\boldsymbol{#2}}}
\newcommand\nonterminal[2][]{\TR[#1]{#2}}
\newcommand\emptyword{\TR{\epsilon}}

\def\psEdge#1#2{\ncdiagg[armA=1cm,angleA=0,angleB=180]{#1}{#2}}

\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[thislevelsep=2.75cm]{\nonterminal{N_1}}{
        \pstree{\nonterminal[edge=\psEdge]{N_1}}{ \terminal{[} }
        \terminal[edge=\psEdge]{[}
  }
}
\end{document}