[ 8 base put
    nextword dup
    $ '' = if
      [ $ '"octal" needs a number after it.'
        message put bail ]
    dup $->n iff
      [ nip swap dip join ]
    else
      [ drop
        char " swap join
        $ '" is not octal.'
        join message put bail ]
    base release ]              builds octal ( [ $ --> [ $ )