(*  Generalized Cantor Sets

    https://twitter.com/wtgowers/status/1170692765342216192

       The Cantor set is the set of all real numbers between 0 and 1
       that have just 0s and 1s in their ternary expansion (i.e.,
       no 2s). It is what you get if you continue this process of
       "removing the middle third" to infinity.
 
    We can generalize this to think of an integer Cantor set to
    consist of all integers that have just 0s and 1s in their base-3
    representation.
 
    We could convert n to base-3 and then check the result digit-by-
    digit.  That's what I do in <is-cantor-number.kln>.  But we don't
    really care about the base-3 value itself, so we can compute the
    answer more efficiently: check each digit in the base-3 value as
    we generate it, rather than make a second pass.  The result is a
    single tail-recursive function.
*)

function main(n : integer): boolean
  if n < 3
     then n < 2
     else main(n / 3) and main(MOD(n, 3))

(* from the Klein library *)
function MOD(m : integer, n : integer): integer
  m - m/n * n