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I have read both:

• proposals/110-avoid-infinite-circuits.txt

If an intermediate server receives more than K relay_early cells,

• proposals/292-mesh-vanguards.txt

Additionally, to avoid linkability, we insert an extra middle node
  after the third layer guard for client side intro and hsdir circuits,
  and service-side rendezvous circuits. This means that the set of
  paths for Client (C) and Service (S) side look like this:

and found that with the extra 4th random middle node added by vanguards circlen, a onion Service using vanguards does not blend in with other services or clients. As each intermediate guards server should passively be able to Monitor by listening receives more than usual relay_early cells with (plus ) extra vanguard relay_early cell and find out node's position in the circuit is guard for that the connecting or is a hidden Service with using vanguards addon enabled easily? please check..