Tree branching starting from specific character of node

Question

I am doing parse trees for propositional logic formulas and I want to have the branches of a tree start from the main connective. Notice that the branching starts at a specific character for each node. Here is an example of what I am looking for:

This was done using phantoms to demonstrate. Here is the code:
documentclass{article}
usepackage{forest}
forestset{
    parse tree/.style={for tree={s sep=3em, minimum size=1.5em}}
}

begin{document}
begin{center}
    begin{forest}
        parse tree
        [phantom{ssssssssssssssssss}$neg (neg P to (Q lor R))$
            [phantom{sssss}$neg P to (Q lor R)$
                [phantom{ss}$neg P$ [$P$]]
                [$Q lor R$
                    [$Q$]
                    [$R$]
                ]
            ]
        ]
    end{forest}
end{center}
end{document}

I'm hoping a solution could include the Forest package, but I am fine with anything that is easy enough to use!

Rmano · Accepted Answer

I am not an expert on forest, but what about smashing the size of part of the equations with mathtools? Like:
documentclass{article}
usepackage{forest}
usepackage{mathtools}
forestset{
    parse tree/.style={for tree={s sep=3em, minimum size=1.5em}}
}

begin{document}
begin{center}
    begin{forest}
        parse tree
        [$negmathrlap{(neg P to (Q lor R))}$
        [$mathllap{neg} to mathrlap{(Q lor R)}$
        [$neg mathrlap{P}$ [$P$]]
                [$Q lor R$
                    [$Q$]
                    [$R$]
                ]
            ]
        ]
    end{forest}
end{center}
end{document}

...the only problem could be the bounding box of the final box, which can (will) be wrong. If this is a feasible solution, you can use a macro:
newcommand{balancemath}[3]{mathllap{#1} #2 mathrlap{#3}}
begin{document}
begin{center}
    begin{forest}
        parse tree
        [$balancemath{}{neg}{(neg P to (Q lor R))}$
        [$balancemath{neg}{to}{(Q lor R)}$
        [$balancemath{}{neg}{P}$ [$P$]]
                [$Q lor R$
                    [$Q$]
                    [$R$]
                ]
            ]
        ]
    end{forest}
end{center}

...to obtain the same result. If needed, the macro can be changed (not trivially) to have a phantom bounding box so that it encompasses the full formula.

Sašo Živanović · Answer

I'm adding this answer even if it does not address OP's main issue (starting the branch below the connective), because (i) it provides a solution for the same problem (logic parse trees) and (ii) I had it already developed for myself.
documentclass{article}

usepackage{forest}

forestset{
  declare toks={left}{},
  declare toks={right}{},
  declare boolean={paren}{false},
  formula as tree/.style={
    formula@as@tree,
    for tree={
      content format={ensuremath{%
        noexpandcolor{lightgray}forestoption{left}%
        noexpandcolor{black}forestoption{content}%
        noexpandcolor{lightgray}forestoption{right}%
        }},
    },
  },
  formula@as@tree/.style={
    % assuming unary or binary branching nodes
    delay={
      for descendants children-first={
        if n=1{
          if={n_children("!u")==1}{
            append content to parent,
          }{
            prepend content to parent,
          }
        }{
          append content to parent,
        },
      },
    },
  },
  prepend content to parent/.style={
    if paren={
      !u.+left/.process={OOOw3}{left}{content}{right}{(##1##2##3)},
    }{
      !u.+left/.process={OOOw3}{left}{content}{right}{##1##2##3},
    },
  },
  append content to parent/.style={
    if paren={
      !u.right+/.process={OOOw3}{left}{content}{right}{(##1##2##3)},      
    }{
      !u.right+/.process={OOOw3}{left}{content}{right}{##1##2##3},
    }
  }, 
}

begin{document}

begin{forest} formula as tree
  [neg
    [rightarrow,paren
      [neg
        [P]
      ]
      [vee,paren
        [Q]
        [R]
      ]
    ]
  ]
end{forest}

end{document}

Tree branching starting from specific character of node

2 Answers

Add your own answers!

Ask a Question