get value for form field

See sample code below-- note two forms with identically named 'phone number' fields. In JS, I want to get the value for 'phone' from the form that the user submitted. (This is a small example -- I have dozens of fields in the real forms... so I don't want to give every single field in each form its own unique ID.)

<form id='form1'>
   phone number: <input name='phone'>
   <input type='submit' onClick="validate('form1');">
</form>
 
<form id='form2'>
   phone number: <input name='phone'>
   <input type='submit' onClick="validate('form2');">
</form>
 
 
then in the JS...
 
<script>
 
function validate(f) {
   // note that f == 'form1' or 'form2'
   // how do I get the value of the 'phone' field from the form the user submitted?
   // I'm using the Prototype library, if it helps...
}
 
</script>

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