add-grace-properties in TabVoice context

[Marc Hohl]

Hello,

I tried to make grace notes in tablature smaller and found the
function add-grace-properties in the lsr.

I tried the following:

[snip]

\version "2.13.1"

example = \relative c {
  \grace e8 c4 d e f
  \appoggiatura e8 c4 d e f
  \acciaccatura e8 c4 d e f
}

\score {
 <<
   \new Staff {
     \new Voice {
       \clef "G_8"
       \example
     }
   }
   \new TabStaff {
     #(add-grace-property 'TabVoice 'TabNoteHead 'font-size -4)
     \new TabVoice {
       \clef tab
       \example
     }
   }
 >>
}

[snip]

and it seems to work, but lilypond complains with

Programmierfehler: cannot find context:
Fortsetzung, die Daumen drücken
TabVoiceProgrammierfehler: cannot find context:
Fortsetzung, die Daumen drücken
TabVoiceVorverarbeitung der grafischen Elemente...
Solving 1 page-breaking chunks...[1: 1 pages]

The output is ok.

Now I have two questions:
1) Apparently I made some mistakes, but I don't understand the error 
messages.
2) Can I automatize the function call so that the user doesn't have to call
  add-grace-property in every new TabVoice?
  I tried to put this in a layout block, but
  \layout {
     \context {
        \TabStaff
        #(add-grace-property 'TabVoice 'TabNoteHead 'font-size -4)
     }
  }
  gives an error message.

Thank you in advance.

Marc