How to get limited multidominance in a forest tree

Question

I would like to have forest trees that allow a very limited kind of multidominance, as shown in the picture below:

The basic idea is that the terminal node a is centred between and dominated by both X nodes.

This was produced in the following hacky way: I made a ternary branching tree with a phantom middle edge so that the terminal was the terminal node of the middle branch and then manually drew multidominant the branches. This works fine, but I'm wondering if there is a less hacky way to do it.

\documentclass{article}
\usepackage[linguistics]{forest}
\forestset{multi/.style={before typesetting nodes={
     prepend={[X,name=X1,tier=x]},
     append={[X,name=X2,tier=x]}}}}
\begin{document}
\begin{forest}
    [A,s sep=0pt,multi
        [X,phantom, tier=x [a,tier=s,name=X ]]
    ]
\draw (X1.south) -- (X.north);
\draw (X2.south) -- (X.north);
\end{forest} 
\end{document}

@marmot Yes perhaps. This is one of those cases where 'less hacky' ≠ 'easy to understand'. — Alan Munn, Sep 15 '18 at 18:22
@cfr https://chat.stackexchange.com/rooms/99513/multidominance-room — Alan Munn, Oct 06 '19 at 00:33
@cfr I actually thought the bounty got applied automatically to the accepted answer if there was only one. But it's officially awarded now. :) — Alan Munn, Oct 14 '19 at 01:12

score 7 · Accepted Answer · answered Oct 06 '19 at 01:54

\documentclass[border=9pt]{standalone}
\usepackage[linguistics]{forest}
\forestset{
  declare toks={multi name}{},
  multi dom/.style={
    if={>On={level}{0}}{}{%
      delay/.process={OOw2}{name}{multi name}{
        insert before={[##2,tier=##1]},
        insert after={[##2,tier=##1]},
        !u.s sep'=0pt,
      },
      before typesetting nodes={
        replace by/.process={OOw2}{name}{multi name}{[##2, phantom, before drawing tree={content=, no edge, phantom=0, typeset node}, tier=##1, delay={append=##1}]}
      },
      edge path'={(!up.parent anchor) -- (.child anchor) -- (!un.parent anchor)},
    },
  },
  multi/.style={
    multi name=#1,
    multi dom,
  },
}
\begin{document}
\begin{forest}
  [A
    [a,multi=X ]
  ]
\end{forest} 
\begin{forest}
  [A
    [a,multi=X ]
    [b [c,multi=C] ]
    [f [g,multi=H] ]
  ]
\end{forest} 
\begin{forest}
  for tree={multi name=J}
  [A
    [a,multi dom ]
    [b [c,multi=C] ]
    [f [g,multi dom] ]
  ]
\end{forest} 
\begin{forest}
  [C [A
      [a, multi=C  ]
  ]
    [B [c,multi=V ]]
  ]
\end{forest}
\end{document}

How to get limited multidominance in a forest tree

1 Answers1

Linked