have you looked in the manual?

while (1) {
my.frame += time_step; // 16 frames per second
if (my.frame > 30) { my.next_frame = 20; } // interpolate to start frame
else { my.next_frame = 0; } // interpolate to FRAME+1
if (my.frame >= 31) { my.frame -= 10; } // if end frame reached, skip back
wait(1);
}

use somewhere in the code
while(!key_cud){wait(1);}
to make it pause