Validate specific input, Javascript

<form action="/cgi-bin/Lib.exe" method=POST name="checks">

     <input type=checkbox name="user1" value="'$NAME'">
     <input type=checkbox name="user2" value="'$NAME'">
     <input type=checkbox name="user3" value="'$NAME'">

<input type="button" value="User 1" onclick="somefunction()">

<form action="".... onsubmit="return somefunction(this);">

function somefunction(f) {
    var user1 = f.user1;
    var user2 = f.user2;
    // etc..
    if(user2.checked) {
        alert("you are not user1");
        return false;
    }
    return true;
}

<form onsubmit="somefunction(this);" ...

function somefunction(f) {
    var user1 = f.user1;
    var user2 = f.user2;
    // etc..
    if(user2.checked) alert("you are not user1");
}

<script language="javascript">