function bta($b, $c)
dim $,$x
 if $c
  for $=-729 to -$
   $x=$x+$*($b=bta($,0))+$*($c=bta($,0))
  next
  $bta=bta($x,0)
else
 do
  $ = $b mod 3
  $c = $/2
  $bta = "" + ($ - 3*$c) + $bta
  $b = ($b+$c) / 3
 until $b=0
endif
endfunction