title: Grant permission to grant only certain roles to a user
body: <p>I have role A and role B, I want to give the permission to some users to give role A to another user, but not role B.
  Actually I'm using <code>GRANT GRANT ANY ROLE TO user</code>, but it allows user to give whatever role he wants.</p>
  
  <p>So basically I need something like <code>GRANT GRANT A TO username</code>.</p>
  
  <p>EDIT: The role that user has to grant to another user, it's not the role he belongs</p>