为什么p_sequencer不指定具体的sequencer也可以调用sequence

1. class base_vseqr extends ovm_virtual_sequencer;
   
2. X_seqr X_seqr_h;
   
3. Y_seqr Y_seqr_h;
   
4. 
   
5. `ovm_sequencer_utils(base_vseqr)
   
6. .....
   
7. endclass
   
8. 
   
9. class A_seq extends ovm_sequence;
   
10. B_seq B_seq_h;
    
11. `ovm_create_on(B_seq_h, p_sequencer);
    
12. B_seq_h.start(p_sequencer);
    
13. endclass

------>>>> `ovm_sequencer_utils(base_vseqr)