Firebase Fan-out update validation in Bolt

Lets assume we have a data structure similar to the one below, and using fan-out update we want to convert an invitation to a user.

• root
  • users
    • KC88nKzEqsx6AlPjrvp
      • email : test1@yahoo.com
      • uid: 19570b6c-afd0-xxxx-xxxx-650be04c5767
    • KCAW6T_au2ROnJcaf85
      • email : test2@yahoo.com
      • uid: 22570b6c-afd0-xxxx-xxxx-650be04c5767
  • invitations
    • KCZndfnKzEqrtr6AlPjrvp
      • email : test3@yahoo.com
    • BYYAW6T_au2Rffeaf85
      • email : test4@yahoo.com

Update code:

var update = {}
update[‘invitations/KCZndfnKzEqrtr6AlPjrvp’] = null
update[‘users/DDd83sx6AlPjrvp’] = {email:test3@yahoo.com, uid: 773492-afd0-344-4535-650be04c5767}

ref.update(update);

How can i write validate rules in Bolt, that will check if:

1. Invitation for such email existed.
2. The newly created user email is equal to email on invitation
3. User uid is his actual id from auth object.