Quickform vaildation rules

I have a Quickform with a validation rule stating that a textarea is "required". That works well. I have three buttons on the form, "Clear", "Submit" and "Cancel". The first is the standard reset button; the latter two are submit buttons. The problem I have: if the "Cancel" button is clicked with the textarea empty, I get the expected validation error. I really want a rule that says "(textarea required) OR (btn='Cancel')". Is there a way of achieving this? -- Keith Edmunds