Centering text spanners under a note

[Maurizio Tomasi]

I am typesetting an old edition of Mozart quartets, and sometimes
dynamics appear in parentheses.  I manage to write these with

-------------------------------------------------------------------
\score {
  \context Staff {
    \notes \relative c' {
      c1_#'(columns (large "(") (dynamic "p") (large ")"))
    }
  }
}
-------------------------------------------------------------------

but the "(p)" string is not rightly centered above the note (it is a
bit at the right because of the parentheses).  I tried with the
"edge-text" of the Text Spanner grob, but without success (brackets
are not shown at all):

-------------------------------------------------------------------
\score {
  \context Staff {
    \notes \relative c' {
      \once \property Voice.TextSpanner
            \override #'edge-text = #'("(" . ")")
      f4_#'(dynamic "p")
    }
  }
}
-------------------------------------------------------------------

What could be the best possible solution?

Many thanks
  Maurizio.


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