: next-iteration ( n -- m )
    dup even? [ 2 / ] [ 3 * 1 + ] if ;

: (chain) ( count n -- count )
    dup 1 = [ drop ] [ next-iteration [ 1+ ] dip (chain) ] if ;

: chain ( n -- count )
    [ 0 ] dip (chain) ;


! (scratchpad) 4 chain .
2