Add some comments

theories/examples/queue/spec_graph.v

@@ -259,7 +259,7 @@ Definition queue_empty_unmatched_enq_strong G : Prop :=
   See more weakenings below.
 
   On the other hand, we can also strengthen it further by changing the
-  conclusion from (d1,d2) ∈ xo (d1,d2) ∈ eco. *)
+  conclusion from [(d1,d2) ∈ xo] to [(d1,d2) ∈ eco]. *)
 Definition queue_FIFO_strong G : Prop :=
   ∀ e1 v1 eV1 e2 d2,
     G.(Es) !! e1 = Some eV1 → eV1.(ge_event) = Enq v1 →
@@ -282,7 +282,8 @@ Definition queue_FIFO G : Prop :=
   .
 
 (* Alternatively, if e2 as an enqueue that has been dequeued but also observed
-  an enqueue e1, then e1 must also have been dequeued. *)
+  an enqueue e1, then e1 must also have been dequeued.
+  This encodes the Yacovet's condition [com^-1; lhb; com; lhb is irreflexive]. *)
 Definition queue_FIFO_weak G :=
   ∀ e1 d1 dV1 e2 d2 eV2,
     (e1, d1) ∈ G.(com) → (e2, d2) ∈ G.(com) →