function vigenere($t,$a,optional $_)

dim $,$w,$p,$k,$l,$z

 for $=1 to 256

  if $>47&$<58|($>64&$<91)|($>96 & $<123 & $t+$a<>ucase($t+$a))

  $l=$l+chr($)

 else

  $a=join(split($a,chr($)),'')

 endif

 next

 for $k=1 to len($t)

  $z=$z+$a

  $=substr($t,$k,1)

  $w= ($=' ') +$w-$w*instr('!.?',$)

  $p=instr($l,$)

  if $p

   $=len($l)

   $=substr($l,1+($+($p-1+(1-2*$_)*($w+instr($l,substr($z,$k,1)))) mod $) mod $,1)

  endif

  $vigenere=$vigenere+$



 next

endfunction