I was thinking the same but using Rebol:

  todo [
      housework
      high: "Clean the house"
      medium: "Wash the dishes"
      medium: "Buy more soap"
  ]

And this is so easy to validate & use via the parse dialect that comes with Rebol.

Here's a validation example:

  todo-rule: [
      set task-name word!
      some [
          set item-priority set-word!
          set item-desc string!
      ]
  ]

  >> parse [bad] todo-rule
  == false
 
  >> parse [incomplete high:] todo-rule
  == false
 
  >> parse [incorrect high: "item1" "erggh!"] todo-rule
  == false
 
  >> parse [housework high: "foo" medium: "bar" medium: "baz"] todo-rule    
  == true