Thanks for illustration! Just check if my understanding is correct. Based on the example above (6 transactions), if Charlie is to approve Alice has 10 as she claims, he needs to trace,
Tx #3 that Alice was given 8 iotas by Genesis (checked)
Tx #6 that Alice was given 2 iotas by Bob, and then Tx #2 that Bob was given 2 iotas by Genesis (checked)
That proves that Alice can spend 10 iotas as all are chased back to Genesis.