thought that we should go under 100, so here it is, 99:
Code:

Function b($)
 do
  If $+0=$
   $b = '' + ($ mod 2) + $b
   $ = $ / 2
  else
   $b = 2*$b + left($,1)
   $ = right($,~)
  endif
 Until $ = .
EndFunction