[2..0d100_000].pump(Void,  // loop n from 2 to 100,000
   collatz,              // generate Collatz sequence(n)
   fcn(c,n){           // if new longest sequence, save length/C, return longest
      if(c.len()>n[0]) n.clear(c.len(),c[0]); n}.fp1(L(0,0)))