title: ERROR: 'Checker 'xor_module_b' not found. Instantiation 'x0_1' must be of a visible checker.'?
body: <p>What is this error 'Checker 'xor_module_b' not found.  Instantiation 'x0_1' must be of a visible checker.'? I am writing verilog code in behavioral model by using module instantiation. While compiling i am getting the error. Portion of code and error is attached.</p>
  
  <pre><code>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
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/ThHnC.jpg" rel="nofollow noreferrer">Error Snapshot</a></p>