Why is prooftrees putting everything on the same line?

Question

I'm trying to do a prooftrees tree like this:

\documentclass[letterpaper, 12pt]{article}
\usepackage{prooftrees}
\begin{document}
\begin{prooftree}
{
    to prove={ A \lor B \lor C \lor D \lor E }
}
    [ A ]
    [ B ]
    [ C ]
\end{prooftree}                                                                                                                     
\end{document}        

But it currently outputs this:

It's all on the same line while I'm trying to get a tree with branches on multiple lines. Can anyone tell me what I'm doing wrong?

EDIT: Tried a forest tree, it doesn't work either. Lines are drawn but the letters are all still crammed like in the picture.

Answer 1

It's a tree, so it needs one root and branches.

For instance

\documentclass{article}
\usepackage{prooftrees}

\begin{document}

\begin{prooftree}
{
    to prove={A \lor B \lor C \lor D \lor E}
}
[A
  [ B ]
  [ C
    [ D ]
  ]
]
\end{prooftree}

\end{document}

Note that, in order to see the real result, you have to look at a PDF output, either running pdflatex or latex+dvips+ps2pdf; the DVI file will not show the correct output, because TikZ/PGF, which prooftrees is based on, uses “specials” that DVI viewers are not able to show.

Answer 2

egreg is correct regarding the solution, but the diagnosis is wrong. The problem is not that the tree has no root. It has a perfectly good root. Moreover, it does not superimpose nodes if compiled with an appropriate engine (e.g. pdfTeX).

\documentclass[border=10pt]{standalone}
\usepackage{prooftrees}
\begin{document}
\begin{prooftree}
{
    to prove={ A \lor B \lor C \lor D \lor E }
}
    [ A ]
    [ B ]
    [ C ]
\end{prooftree} 
\end{document}

produces

Now, admittedly, this probably is not what you meant or want, but it is reasonably sensible output, given the input.

Suppose we instead try

\begin{forest}
    [ A ]
    [ B ]
    [ C ]
\end{forest}

which produces

This is because Forest takes [ A ] to be the root and, hence, the entire tree. Everything after the tree specification is passed to TikZ, but there are no relevant commands here, so [ B ] and [ C ] are parsed as text. Since we are inside a tikzpicture environment, we are using the null font and the text is ignored. Hence, we effectively have

\begin{forest}
  [ A ]
\end{forest}

The reason that the prooftree environment behaves differently is because the tree specified by the user is actually a sub-tree of the tree the environment typesets. prooftree embeds the user's tree specification in a larger tree which includes the line numbers, justifications and 'to prove' line, as appropriate. Hence, the inner forest environment reads

  [ A ]
  [ B ]
  [ C ]

as part of the tree specification, so these end up being nodes in the tikzpicture and get typeset in the normal font, which is switched back by TikZ for node text.

The user provided root in a prooftree is 'never' the root of the typeset tree. I'm not sure it is possible to make a prooftree attempt to set a rootless tree and, although I would not wish to rule this possibility out, I can say that it is most unlikely that anybody would do so by accident.