Refer to title of direct superior section in an inferior section

Question

General

I know there are question about "how to refer to a specific title" however I need a way to generally refer to the superior section's title or to the title of the current section of a specific level (similar to referring to a section's counter using \thesection).

My documentclass is article.

Problem

I have a tree like document where the title of each section describes an element in the tree. The section titles should be the path to the corresponding element.

However I don't want to specify the full path to the element in the section command (\subsubsection{A/B1/C}) as this can cause a lot of refactoring work when an element in the path changes.
I would prefer to only specify the leaf element (the last or tail element) of the path while the head is automatically deduced from the superior section. Something like:

% How should \currentsectionname and \currentsubsectionname be defined?
\newcommand{\treesubsection}[1]{\subsection{\currentsectionname /#1}}
\newcommand{\treesubsubsection}[1]{\subsubsection{\currentsubsectionname /#1}}

\section{A}
\treesubsection{B1}
\treesubsubsection{C}

I tried the solutions from this question however they only seem to work when used within a section's body but not within the title:

• The nameref solution gives me TeX capacity exceeded, sorry [input stack size=5000].
• The titleref solution duplicates the leaf title (i.e. results in for example A/B1/B1 instead of A/B1).
• The zref-titleref solution puts a "currentname" instead of the head and also duplicates the leaf.

Question

How can I refer to the superior section's title or to the title of the current section of a specific level, similar to \thesection, \thesubsection, ... ?
In addition I want to use the feature that I can choose a custom section title for the TOC, as it is possible for \section[custom toc title]{title}. Actually the custom title should always be the leaf name.

EDIT (what I tried specifically)

nameref

\usepackage{nameref}
\makeatletter
\newcommand*{\currentname}{\@currentlabelname}
\makeatother

\newcommand{\treesubsection}[1]{\subsection{\currentname /#1}}
\newcommand{\treesubsubsection}[1]{\subsubsection{\currentname /#1}}

titleref

\usepackage{titleref}
\makeatletter
\newcommand*{\currentname}{\TR@currentTitle}
\makeatother

\newcommand{\treesubsection}[1]{\subsection{\currentname /#1}}
\newcommand{\treesubsubsection}[1]{\subsubsection{\currentname /#1}}

zref-titleref

\usepackage{zref-titleref}
\makeatletter
\newcommand*{\currentname}{\zref@getcurrent{title}}
% \newcommand*{\currentname}{\zref@titleref@current}
\makeatother

\newcommand{\treesubsection}[1]{\subsection{\currentname /#1}}
\newcommand{\treesubsubsection}[1]{\subsubsection{\currentname /#1}}

Answer 1

Using \newcommand{\currentname}{\@currentlabelname} has no effect. It would refer to the current value of \@currentlabelname or is even undefined. In fact, the \@currentlabelname must be stored in an expansion step to another macro after the section title has been set, not in the argument.

Please note that the current definition of \tree... misses the possibility of using \treesection[short toc title]{long title}.

\documentclass{article}

\usepackage{nameref}
\makeatletter

\newcommand{\treesection}[1]{\section{#1}
 \edef\currentsectionname{\@currentlabelname}}

\newcommand{\treesubsection}[1]{\subsection{\currentsectionname/#1}\edef\currentsubsectionname{\@currentlabelname}}
\newcommand{\treesubsubsection}[1]{\subsubsection{\currentsubsectionname /#1}\edef\currentsubsubsectionname{\@currentlabelname}}
\makeatother


\begin{document}

\treesection{A}
\treesubsection{B1}
\treesubsection{B2}
\treesubsubsection{C1}


\treesection{Z}
\treesubsection{B1}
\treesubsection{B2}
\treesubsubsection{C1}


\end{document}

Update 'Automatic' definition of a tree level and declaration of the \current...name with xparse:

\documentclass{book}

\usepackage{nameref}

\usepackage{xparse}

\setcounter{secnumdepth}{3}

\makeatletter

\NewDocumentCommand{\DefineTreeLevel}{om}{%
  \expandafter\NewDocumentCommand\csname tree#2\endcsname{som}{%
    \IfBooleanTF{##1}{%
      \csname #2*\endcsname{##3}
    }{%
      \def\@@tempa@@{}%
      \IfValueT{#1}{%
        \def\@@tempa@@{\csname current#1name\endcsname/}
      }
      \IfValueTF{##2}{%
        \csname #2\endcsname[##2]{\@@tempa@@##3}%
      }{%
        \csname #2\endcsname{\@@tempa@@##3}%
      }%
      \expandafter\edef\csname current#2name\endcsname{\@currentlabelname}%
    }%
  }%
}

\makeatother


\DefineTreeLevel{chapter}
\DefineTreeLevel[chapter]{section}
\DefineTreeLevel[section]{subsection}
\DefineTreeLevel[subsection]{subsubsection}


\begin{document}

\treechapter{Gamma}
\treesection{A1}
\treesubsection{B1}
\treesubsection{B2}
\treesubsubsection{C1}


\treesection{Z}
\treesubsection{B1}
\treesubsection{B2}
\treesubsubsection{C1}


\end{document}

Update

ToC - related stuff with some screenshot

\documentclass{article}

\usepackage{nameref}

\usepackage{xparse}

\setcounter{secnumdepth}{3}

\makeatletter

\NewDocumentCommand{\DefineTreeLevel}{om}{%
  \expandafter\NewDocumentCommand\csname tree#2\endcsname{som}{%
    \IfBooleanTF{##1}{%
      \csname #2*\endcsname{##3}
    }{%
      \def\@@tempa@@{}%
      \IfValueT{#1}{%
        \def\@@tempa@@{\csname current#1name\endcsname/}
      }
      \IfValueTF{##2}{%
        \csname #2\endcsname[##2]{\@@tempa@@##3}%
      }{%
        \csname #2\endcsname{\@@tempa@@##3}%
      }%
      \expandafter\edef\csname current#2name\endcsname{\@currentlabelname}%
    }%
  }%
}

\makeatother


\DefineTreeLevel{section}
\DefineTreeLevel[section]{subsection}
\DefineTreeLevel[subsection]{subsubsection}


\begin{document}
\tableofcontents
\treesection{A1}
\treesubsection[B1]{B1}
\treesubsection{B2}
\treesubsubsection[C1]{C1}


\treesection{Z}
\treesubsection{B1}
\treesubsection{B2}
\treesubsubsection{C1}


\end{document}

Next update, recursive \@current.... names:

\documentclass{article}

\usepackage{nameref}

\usepackage{xparse}

\setcounter{secnumdepth}{3}

\makeatletter

\NewDocumentCommand{\DefineTreeLevel}{om}{%
  \expandafter\NewDocumentCommand\csname tree#2\endcsname{som}{%
    \IfBooleanTF{##1}{%
      \csname #2*\endcsname{##3}%
    }{%
      \gdef\@@tempa@@{}%
      \IfValueT{#1}{%
        \def\@@tempa@@{\csname current#1name\endcsname/}
      }%
      \IfValueTF{##2}{%
        \csname #2\endcsname[##2]{\@@tempa@@##3}%
        \expandafter\edef\csname current#2name\endcsname{\@@tempa@@\@currentlabelname}%
      }{%
        \csname #2\endcsname{\@@tempa@@##3}%
        \expandafter\edef\csname current#2name\endcsname{\@currentlabelname}%
      }%
    }%
  }%
}

\makeatother


\DefineTreeLevel{section}
\DefineTreeLevel[section]{subsection}
\DefineTreeLevel[subsection]{subsubsection}


\begin{document}
\tableofcontents
\treesection{A1}
\treesubsection[B1]{B1}
\treesubsubsection[C1]{C1}
\treesubsection[B2]{B2}
\treesubsubsection[C2]{C2}
\treesubsubsection[C3]{C3}


\treesection{Z}
\treesubsection{B1}
\treesubsection{B2}
\treesubsubsection{C1}


\end{document}