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
  $bta=""+($-$/2*3)+$bta
  $b=($b+$/2)/3
 until $b=0
endif
endfunction