Flash 6 " set mask"

how do you use the “set mask” command with a movieclip on the main scene?

place 2 movie clips on the stage so that parts of them overlap. name them ‘mc1’ and ‘mc2’. make the root timeline 20 frames long.
in frame 1:
// ‘mc1’ will be masked by ‘mc2’…
in frame 5:
// unmask ‘mc1’…
in frame 9:
// ‘mc2’ will be masked by ‘mc1’…
in frame 14:
// unmask ‘mc2’…

you will see the movie clips mask then unmask upon testing the movie.