问错误：“检查器”“xor_module_b”未找到。实例化'x0_1‘必须是一个可见的检查器。’？

EN

module CSSA4_4bit_modified_b(s,cin,g,G,GP,a,b);
input cin,g,G,GP;
input [7:0] a,b;
output wire [7:0] s;
wire [6:0] c0;
wire [3:0] c1;
wire [2:0] pro;
wire [7:0] s0;
wire [3:0] s1;

always@(a,b,cin,g,G,GP)
begin
//Subblock 1
//Sum bit 0
xor_module_b x0_1(.a(a[0]), .b(b[0]),.s0(s0[0]));
xor_module_b x0_2(.a(s0[0]),.b(cin), .s0(s[0]));
and_logic_b  a0 (.a(s0[0]), .b(cin), .out(pro[0]));
//end

//Sum bit 1
FA_b         FA_b1(.a(a[1]), .b(b[1]),  .c(g),.sum(s0[1]),.cout(c0[0]));
xor_module_b x1 (.a(s0[1]),.b(pro[0]),.s0(s[1]));
and_logic_b  a1 (.a(s0[1]),.b(pro[0]),   .out(pro[1]));
//end

//Sum bit 2
FA_b         FA_b2(.a(a[2]), .b(b[2]),  .c(c0[0]),.sum(s0[2]),.cout(c0[1]));
xor_module_b x2 (.a(s0[2]),.b(pro[1]),.s0(s[2]));
and_logic_b  a2 (.a(s0[2]),.b(pro[1]),.out(pro[2]));
//end.......continued
//Sum bit 7
FA_b FA_b1_7_1(.a(a[7]),.b(b[7]),.c(c0[5]), .sum(s0[7]),.cout(c0[6]));
FA_b FA_b1_7_2(.a(a[7]),.b(b[7]),.c(c1[2]), .sum(s1[3]),.cout(c1[3]));
sum_select_mux_b M1_7(.Sum(s[7]),.Sum0(s0[7]),.Sum1(s1[3]),.C8k(cin));
//End of subblock 2
//End of CSSA 4-4 bit
end
endmodule