I would like to add a rectangle upon a subtree like in the following picture. How to do that ?
Here is the forest
code.
\documentclass{article}
\usepackage{forest}
\useforestlibrary{linguistics}
\begin{document}
\pagestyle{empty}
\begin{forest}
for tree = {
sn edges,
grow'=0,
l=2.5cm,
s sep=0.2cm,
anchor=west,
child anchor=west}
[
[O
[R
[T]
]
[T
[R]
]
]
[R
[O
[T]
]
[T
[O]
]
]
[T
[O
[R]
]
[R
[O]
]
]
]
\end{forest}
\end{document}