to rpn
  if eof? [stop]
  make "s reverse rl
  if not empty? :s ~
    [ make "r catch "err [eval]
      pr catch "err
        [ifelse empty? :s [:r] [(throw "err "error)]] ]
  rpn
end

to eval
  (local "x "y "t)
  if not empty? :s ~
    [ make "t first :s  make "s bf :s
      if member? :t [+ - * /] ~
        [ make "y eval  make "x eval
          op run (list :x :t :y) ]
      if number? :t [op :t] ]
  (throw "err "error)
end