digits([arg]) := block(
   [n: first(arg), b: if length(arg) > 1 then second(arg) else 10, v: [ ], q],
   do (
     [n, q]: divide(n, b),
     v: cons(q, v),
     if n=0 then return(v)))$

binary(n) := simplode(digits(n, 2))$
binary(9000);
/*
                                10001100101000
*/