Puppet users - Jan 2014 - only subclasses can modify ?

I'm trying to modify certain users, created with by calling the define 
users::local.

I tried to do it, by calling users::local::modify - but then puppet 
complains that only subclasses may modify.. :(

What am I doing wrong here?

Code:

define users::local 
($uid,$gid,$realname,$groups=[],$pass='',$sshkey="",$shell="/bin/bash",$home="/home/$title")
{
                user { $title:
                        ensure  =>      "present",
                        uid     =>      $uid,
                        gid     =>      $gid,
                        groups  =>      $groups,
                        shell   =>      $shell,
                        home    =>      $home,
                        comment =>      $realname,
                        password =>     $pass ? {
                        '' => undef,
                        default => $pass
                        },
                        managehome =>   true,
                }

                if ( $sshkey != "" ) {
                 ssh_authorized_key { $title:
                         ensure  =>      "present",
                         type    =>      dsa,
                         key     =>      "$sshkey",
                         user    =>      "$title",
                         require =>      User["$title"],
                         name    =>      "$title",
                 }
                }
}

define users::local::modify 
($uid='',$gid='',$realname='',$groups=[],$pass='',$sshkey="",$shell="/bin/bash",$home="/home/$title")
{
  if (  ! empty($groups) ) {
    User[$title] { groups => $groups }
  }
}

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