Tikz Tree with added notes

Question

I need to add notes where "Write Text Here" is. I do not want it to be part of the node. Is there a way to add notes next to a node.

Below is my code;

Thank you

\tikzstyle{every node}=[draw=white,thick,anchor=west]
\tikzstyle{selected}=[dashed,fill=red!30]
\tikzstyle{optional}=[dashed,fill=gray!50]
\begin{tikzpicture}[%
  grow via three points={one child at (0.5,-0.7) and
  two children at (0.5,-0.7) and (0.5,-1.4)},
  edge from parent path={(\tikzparentnode.south) |- (\tikzchildnode.west)}]
  \node[fill=purple] {\color{white}\textbf{{Main Theorem}}}
    child { node {Lemma1: {\small Write Text Here}}}        
    child { node {Lemma2:} 
        child {node {\small Write Text Here}}}
    child { node {Lemma3: {\small n}}}
    child { node {Lemma4: {\small Write Text Here}}}
    child { node {Lemma5: {\small Write Text Here}}}
    child { node {Lemma6: {\small Write Text Here}}}
    child { node [selected]{Lemma7: {\small Write Text Here}}}
    child { node [selected]{Lemma8: {\small Write Text Here}}}              
    %child [missing] {}             
    child { node [selected]{Lemma9: {\small Write Text Here}}}
    ;
\end{tikzpicture}  

Answer 1

Excessively verbose, in my view, when options such as Forest or tikz-qtree exist, but for those who enjoy typing:

\documentclass[border=10pt,tikz,multi]{standalone}
\usetikzlibrary{trees}
\begin{document}
\tikzset{% \tikzstyle is deprecated
  every node/.style = {draw=white, thick, anchor=west},
  selected/.style = {dashed, fill=red!30},
  optional/.style = {dashed, fill=gray!50},
}
\begin{tikzpicture}[%
  grow via three points = {one child at (0.5,-0.7) and two children at (0.5,-0.7) and (0.5,-1.4)},
  edge from parent path={(\tikzparentnode.south) |- (\tikzchildnode.west)},
  every label/.append style={font=\small},
  ]
  \node[fill=purple] {\color{white}\textbf{{Main Theorem}}}
  child { node [label=right:Write Text Here] {Lemma1: }}
  child { node {Lemma2:}
    child {node [label=right:Write Text Here] {}}}
  child [missing] {}
  child { node  {Lemma3: {\small n}}}
  child { node [label=right: Write Text Here] {Lemma4: }}
  child { node [label=right:Write Text Here] {Lemma5: }}
  child { node [label=right:Write Text Here] {Lemma6: }}
  child { node [label=right:Write Text Here] [selected]{Lemma7: }}
  child { node [label=right:Write Text Here] [selected]{Lemma8: }}
  child { node [label=right:Write Text Here] [selected]{Lemma9: }}
  ;
\end{tikzpicture}
\end{document}

produces

It will come as no surprise that I would not do this but instead something like the following.

\documentclass[border=10pt,tikz,multi]{standalone}
\usepackage[edges]{forest}
\begin{document}
\tikzset{% \tikzstyle is deprecated
  every node/.style = {draw=white, thick},
  selected/.style = {dashed, fill=red!30},
  optional/.style = {dashed, fill=gray!50},
  every label/.append style={font=\small},
}
\begin{forest}
  forked edges,
  for tree={%
    folder,
    draw,
    grow'=0
  }
  [\textbf{Main Theorem}, white, fill=purple
    [Lemma1:, label=right:Write Text Here]
    [Lemma2:
      [, label=right:Write Text Here]
    ]
    [Lemma3: {\small n}]
    [Lemma4:, label=right:Write Text Here]
    [Lemma5:, label=right:Write Text Here]
    [Lemma6:, label=right:Write Text Here]
    [Lemma7:, label=right:Write Text Here, selected]
    [Lemma8:, label=right:Write Text Here, selected]
    [Lemma9:, label=right:Write Text Here, selected]
  ]
\end{forest}
\end{document}

This yields