Javascript function to change form fields

Hi - I wonder if anyone can help with what is hopefully a simple javascript request - if such a thing exists!

What I want to do is to examine a pair of fields - lets call them F1 and F2 - both of which are select boxes and both of which can have the values of "Yes" and "No".

If F1 is set to "Yes" then F2 must be set to "No" and vice versa. Now, I could probably do this using the form name and form field names and so on via the onchange event, but it struck me that there must be some way of writing a generic script that would allow you to pass the relevant parameters and cause the setting of the companion field to be the opposite.

So, the script being called doesn't know the form name (or ID) and doesn't know the field names/ids and doesn't know the value pairs - these all being passed at onchange time.

Does anyone know how I pass the fields to the JS function? For example if I have:

<form name="form1">
  <select name="f1" OnChange="change_values(this)">
      <option value="Yes">Yes</option>
      <option value="No">No</option>
  </select>
  <select name="f2" OnChange="change_values(this)">
      <option value="Yes">Yes</option>
      <option value="No">No</option>
  </select>

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And the javascript...

 <script type="text/javascript">
  function change_values(form_field)
  {
  if (formfield.form.f1.value == "Yes"){
  formfield.form.f2.value = "No"
  }
else{
  formfield.form.f2.value = "Yes"
  }
  }
  </script>

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But I don't want to hard code F1 or F2 or "Yes" or "No" and my JS is pretty basic!

Many thanks