Problem Statement

Show a loop which prints random numbers (each number newly generated each loop) from 0 to 19 (inclusive). If a number is 10, stop the loop after printing it, and do not generate any further numbers.
Otherwise, generate and print a second random number before restarting the loop.
If the number 10 is never generated as the first number in a loop, loop forever.

Let's start with the solution:

integer
main(void)
{
    integer a, b;

    while (1) {
        a = drand(19);
        o_integer(a);
        o_byte('\n');
        if (a == 10) {
            break;
        }

        b = drand(19);
        o_integer(b);
        o_byte('\n');
    }

    return 0;
}