covergroup cg_multi_layer (ref int layer0_index, int layer1_index, input int size);
      cp_layer0: coverpoint layer0_index {
        bins id[]={[0:size]};
      }
      cp_layer1: coverpoint layer1_index {
        bins id[]={[0:size]};
      }
      cx_layers : cross cp_layer0, cp_layer1;
    endgroup
 

在new function中，准备所需要的sequence queue， 然后random

seq_q.push_back("xxx0"); ...

layer[0] = $urandom_range(seq_q.size()-1, 0);
layer[1] = $urandom_range(seq_q.size()-1, 0);

cg_multi_layer = new(layer[0], layer[1], seq_q.size()-1);  // 也可以使用string表示