blank lines - how to ?

[Dan Doner]

I'm trying to print lesson sheets for my Son that contain blank lines
and just portions of blank lines.  Please bear with me, I'm not a
musician.

Printing any type of music isn't a problem, but printing blank sheet
music or portions
of blank music is a problem.  I've scoured the reference but can't find
anything to help, although I imagine I've missed it.  Any pointers would
be most appreciated.

Here is what I'm doing:

melody = \notes \relative c'' {
        \clef violin;
        \property Staff.instrument = "piano"

\repeat "volta" 2 {

        c d e f | g a b c | s s s s | c,, d e | f g a b | c r

%                              ^^^^^^^^  this is where I want a blank
measure, not just
% two vertical bars right next to each other.

        }
}
accompany = \notes \relative c {
        \clef "bass";

        \repeat "volta" 2 {

        s
% I'd also like for the base clef to be printed, but blank - how to do
this????

        }
}

global = \notes {
        \time 4/4;
}
\score{
        \notes <
%               \context Staff=i s1
                \context Lyrics=top s1
                \context GrandStaff <
                        \context Staff=ii \repeat semi 1 <
\global\melody >
                        \context Staff=iii \repeat semi 1 <
\global\accompany >
                >
        >
        \paper{
                gourlay_maxmeasures = 14.0;
        }
        \midi{
                \tempo 4 = 120 ;
        }
}