Avoid a string in a TabStaff

[Davide Liessi]

Hi.

In a passage of a guitar piece that I'm engraving there is the need to
avoid the use of the second string.
I could explicitly mark all affected notes with the correct string
number, but I'd prefer to have an automatic way to avoid that string.

Here's an example:

%%% BEGIN
\version "2.18.0"

<<
  % Default notation:
  \new TabStaff {
    a b gis'
  }

  % What I want:
  \new TabStaff {
    a b\3 gis'
  }
>>
%%% END

Searching the manual I found the defaultStrings property of
Tab_note_heads_engraver and the regression test
tablature-default-strings.ly, but this does not help:

%%% BEGIN
\version "2.18.0"

\new TabStaff \with {
  defaultStrings = #'(3 1)
} {
  a b gis'
}
%%% END

Is there a way to avoid the use of one string but still use the lowest
position possible for notes on other strings?

Best wishes.
Davide