Modifying user roles

[jobidon]

I tried the following:

function test() {
    var that=this;
    //currently logged in as a user with role = '_admin'
    this.DB.signup("batman", "pwd", {metadata: {roles: ['aaa','bbb']}})
            .then(function(r) {
                console.log("Create user: "+r.ok);
                return that.DB.putUser("batman", {metadata: {roles: ['ccc']}})
            }).then(function(r) {
                console.log("Modified user: "+r.ok);        //Modified user: true
                return that.DB.getUser("batman");
            }).then(function(r) {
                console.log("Resulting roles: "+r.roles);   //Resulting roles: ccc,bbb
                return r;
            }).catch(console.log);
    }
}

I was expecting the resulting log to indicate that the resulting role was 'ccc', however, it looks like the first element of the array was replaced by the first element of the new array, but the rest of the array remained unchanged.
How do I go about removing the rest of the roles other than with a: {role: ["","",......]}?

As a side note, how do I delete a user? Is there a deleteUser(name, [callback]) or equivalent?
Thanks for your input!