For a simple example: I have a song with the verses just by a solo voice and piano. In the refrain, there joins a string quartet. I have not found a way to make the string's lines appear only in the refrain, I must add them from the beginning. I have tried a second "staves" statement at the beginning of the refrain, but it muddles up everything, both in abc-explorer and in the output of abcm2ps. Of course I have them from the beginning, and fill them up with an endless series of x'es but that is far from elegant. What is the proper way to do this, i.e. make the strings' lines appear only where there are wanted, in the refrain?