pestanaPrivate = \markup {
  \fontsize #-2
  \combine
    \roman C
    \translate #'(0.6 . -0.4) \draw-line #'(0 . 2.0)
}

pestanaSingle =
  #(define-music-function (fret-number music) (string? ly:music?)
     #{
       $music ^\markup { \pestanaPrivate \fontsize #-2 $fret-number }
     #})

pestanaSpan =
  #(define-music-function (fret-number) (string?)
     #{
       \once \override TextSpanner.font-shape = #'upright
       \once \override TextSpanner.style = #'solid
       \once \override TextSpanner.bound-details.left.stencil-align-dir-y = #CENTER
       \once \override TextSpanner.bound-details.right.text = \markup { \draw-line #'(0 . -.5) }
       \once \override TextSpanner.bound-details.left.text = \markup { \pestanaPrivate \fontsize #-2 $fret-number }
     #})

corda =
  #(define-music-function (string-number) (string?)
    #{
      \once \override TextSpanner.style = #'solid
      \once \override TextSpanner.font-size = #-5
      \once \override TextSpanner.bound-details.left.stencil-align-dir-y = #CENTER
      \once \override TextSpanner.bound-details.right.text = \markup { \draw-line #'( 0 . -.5) }
      \once \override TextSpanner.bound-details.left.text = \markup { \circle \number $string-number }
    #})