biblatex : Book with the same title as MVBook

Question

So, question completely changed because the comments got me on the right track, and updated again (the example is a bit harder now, I think).
But maybe I'm not using MVBook correctly ?

Here we go: In the case of a multi-volume book with volumes that have the same titles to which just a volume number is added, I'm looking for an elegant solution. The standard cite commands do not seem to print the volume number, leading to ambiguous citations if the titles of the volumes are the same.

\documentclass{memoir}
\usepackage[T1]{fontenc}

\makeatletter
%% Biblatex %%
\usepackage[style=verbose-trad2,%
related=true,%
backend=biber]%
{biblatex}% 


\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}

@MVBook{Feynman_Lectures,
  author =   {Richard P. Feynman AND Robert B. Leighton AND Matthew Sands},
  title =    {The Feynman lectures on physics},
  publisher =    {Addison-Wesley Publishing Company},
  volumes = {3},
  year =     1977,
  address =      {Reading, MA.}
}

@Book{Feynman_LecturesI,
  title =    {The Feynman lectures on physics},
  subtitle = {mainly mechanics, radiation and heat},
  volume = {1},
  crossref = {Feynman_Lectures}
}

@Book{Feynman_LecturesII,
  title =    {The Feynman lectures on physics},
  subtitle = {mainly electromagnetism and matter},
  volume = {2},
  crossref = {Feynman_Lectures}
}

\end{filecontents}
\addbibresource{\jobname.bib}

\makeatother


\begin{document}

\title{Titre}


\author{A. Teur}

\maketitle

First volume\footcite{Feynman_LecturesI}

Second volume\footcite{Feynman_LecturesII}

And the first again\footcite{Feynman_LecturesI}


\end{document}

The third citation reads :

Idem, The Feynman lectures on physics, op. cit.

Which is clearly not what it should be. I think one should force citations of books that are part of a MVBook to have the volume number printed each time the title gets printed, but only if the overall title and the volume title are the same.

I could add a "I", "II", etc to the title field of each of the volumes, but this looks redundant in the first citation of a volume :

Richard P. Feynman, Robert B. Leighton, and Matthew Sands. The Feynman lectures on physics. Vol. 1: The Feynman lectures on physics I. mainly mechanics, radiation and heat. 3 vols. Reading, MA.: Addison-Wesley Publishing Company, 1977.

Answer 1

The relevant macro for short subsequent citations in verbose-trad2 is cite:title. We just modify it to include the volume if maintitle and title coincide.

\renewbibmacro*{cite:title}{%
  \printtext[bibhyperlink]{%
    \printfield[citetitle]{labeltitle}%
    \iffieldsequal{maintitle}{title}
      {\setunit{\addcomma\space}%
       \printfield{volume}}
      {}
    \setunit{\nametitledelim}%
    \bibstring[\mkibid]{opcit}}}

MWE

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[style=verbose-trad2,
backend=biber]
{biblatex}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}

@MVBook{Feynman_Lectures,
  author =   {Richard P. Feynman AND Robert B. Leighton AND Matthew Sands},
  title =    {The Feynman lectures on physics},
  publisher =    {Addison-Wesley Publishing Company},
  volumes = {3},
  year =     1977,
  address =      {Reading, MA.}
}

@Book{Feynman_LecturesI,
  title =    {The Feynman lectures on physics},
  subtitle = {mainly mechanics, radiation and heat},
  volume = {1},
  crossref = {Feynman_Lectures}
}

@Book{Feynman_LecturesII,
  title =    {The Feynman lectures on physics},
  subtitle = {mainly electromagnetism and matter},
  volume = {2},
  crossref = {Feynman_Lectures}
}

\end{filecontents*}
\addbibresource{\jobname.bib}

\renewbibmacro*{cite:title}{%
  \printtext[bibhyperlink]{%
    \printfield[citetitle]{labeltitle}%
    \iffieldsequal{maintitle}{title}
      {\setunit{\addcomma\space}%
       \printfield{volume}}
      {}
    \setunit{\nametitledelim}%
    \bibstring[\mkibid]{opcit}}}

\begin{document}
First volume\footcite{Feynman_LecturesI}.

Second volume\footcite{Feynman_LecturesII}.

And the first again\footcite{Feynman_LecturesI}.
\end{document}