function allwords ()
  local line = io.read()    -- current line
  local pos = 1             -- current position in the line
  return function ()        -- iterator function
    while line do           -- repeat while there are lines
      local s, e = string.find(line, "%w+", pos)
      if s then      -- found a word?
        pos = e + 1  -- update next position
        return string.sub(line, s, e)   -- return the word
      else
        line = io.read()    -- word not found; try next line
        pos = 1             -- restart from first position
      end
    end
    return nil            -- no more lines: end of traversal
  end
end