How does biblatex check for equal annotator, introduction's author, etc.?

Question

Biblatex is able to check whether a name appears in different fields and is able to adapt the reference showed. For instance, if translation and annotations are made by the same person, biblatex creates a reference which unifies both data with a string such as Trans. and annot. by [person X]. If I am not wrong this is achieved inside the bibmacro translator+othersstrg (defined in biblatex.def) using the macro ifnamesequal which checks for two names being equal.

Unfortunately in case of equal fields annotator and introduction no check is made. How could I achieve that biblatex does not show the name Michel García twice in the following MWE? I.e. How to force biblatex to check for these two fields being equal?

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}

\usepackage[style=verbose-trad2]{biblatex}
\addbibresource{stack.bib}
\begin{document}
\cite{repertorio1972}

\printbibliography
\end{document}

And this is the bibliographical entry:

@Book{repertorio1972,
title =        {Repertorio de príncipes de España},
year =         1972,
annotator =    {García, Michel},
introduction = {García, Michel},
publisher =    {CSIC},
location =     {Jaén}
}

Answer 1

These checks are indeed done via bibliography macros. Combined strings are defined in the localization modules (lbx files). The default definitions are demonstrative rather than exhaustive; they don't account for all types of overlap in authors of annotations, introductions, commentaries, forewards and afterwords. However you can modify the relevant macros from biblatex.def to achieve the output you want. For example:

\NewBibliographyString{withannotatorin}
\DefineBibliographyStrings{english}{%
  withannotatorin = {with annots\adddot and intro\adddot}}

\newbibmacro*{withannotatorin}{%
  \ifboolexpr{ test {\ifnameundef{annotator}} or test {\ifnameundef{introduction}}
    or not test {\ifnamesequal{annotator}{introduction}} }
    {}
    {\bibstring{withannotatorin}%
     \setunit{\addspace}%
     \printnames[withannotator]{annotator}%
     \clearname{annotator}%
     \clearname{introduction}}}

\renewbibmacro*{withothers}{%
  \usebibmacro{withcommentator}%
  \clearname{commentator}%
  \newunit
  \usebibmacro{withannotatorin}%
  \usebibmacro{withannotator}%
  \clearname{annotator}%
  \newunit
  \usebibmacro{withintroduction}%
  \clearname{introduction}%
  \newunit
  \usebibmacro{withforeword}%
  \clearname{foreword}%
  \newunit
  \usebibmacro{withafterword}%
  \clearname{afterword}}