function np = nullpart(parts, barlines)
nullindices = ones(size(barlines));
for iPart = 1:length(parts)
    for iIndex = 1:length(parts(iPart).indices)
        nullindices(parts(iPart).indices(iIndex)+(0:parts(iPart).n-1)) = 0;
    end
end
np.n = 1;
np.indices = find(nullindices);
np.letter = '-';
np.level = 0;