procedure main()
  ops := "+-*/";  ws := ' \t'
  while stk := [] & line := " " || read() || " " do  {
    line ? while tab(many(ws)) & x:=tab(upto(ws)) do  {
             put(stk,real(x)) |
              (1=*x & find(x,ops) &
               t:=pull(stk) & put(stk,proc(x,2)(pull(stk),t))) |
              (write("error") & break next)
           }
    write((1=*stk & pull(stk)) | "error")
  }
end