In reply to eda2k4:
A1: Yes, ##delay seq_b is the same as 1'b1 ##delay seq_b
A2: $past is relative to the current cycle when the expression gets evaluated.
A3: Assuming there is no rst, (A == $past(B))gets evaluated at t2, t3, t4, etc. using the value of B at t1, t2, t3, etc. respectively. In other words, the value of At is compared to the value of Bt-1.