function ipr0 = append(ipr0, ipr1)
    % append particles from one reconstruction to another
    for f = 1 : length(ipr0.pprops)
        field   = ipr0.pprops{f};
		ipr0.(field) = cat(2, ipr0.(field), ipr1.(field));
		assert(isreal(ipr0.(field)), '%s is not real', field);
    end
end