title: Net, which fans out, cannot be assigned more than one value
body: <p>I am trying to design an 8-bit multiplier based on 4-bit multiplier. so this is my code:</p>
  <pre><code>module _8bit_multiply(a, b, q);
  
  input [7:0] a;
  input [7:0] b;
  
  output [15:0] q;
  
  wire [7:0] q0;
  wire [7:0] q1;
  wire [11:0] q2;
  wire [11:0] q3;
  wire [7:0] x0;
  wire [11:0] x1;
  wire [7:0] sum0;
  wire [12:0] sum1;
  wire [12:0] sum2;
  wire co0;
  wire co1;
  wire temp;
  
  _4bit_multiply m1(a[3:0], b[3:0], q0);
  
  assign q[3:0] = q0[3:0];
  assign x0[3:0] = q0[7:4];
  
  _4bit_multiply m2(a[7:4], b[3:0], q1);
  
  nbit_adder s1(x0, q1, 0, sum0, co0);
  
  _4bit_multiply m3(a[3:0], b[7:4], q2);
  _4bit_multiply m4(a[7:4], b[7:4], x1);
  
  assign q3[11:4] = x1[7:0];
  
  nbit_adder s2(q2, q3, 0, sum1, co1);
  
  nbit_adder s3(sum0, sum1, co0, sum2, temp);
  nbit_adder s4(sum2, 0, co1, sum2, temp);
  
  assign q[15:4] = sum2[12:0];
  
  endmodule
  </code></pre>
  <p>then I get this error:</p>
  <blockquote>
  <p>Error (12014): Net &quot;sum2[11]&quot;, which fans out to &quot;q[15]&quot;, cannot be
  assigned more than one value</p>
  <p>Error (12015): Net is fed by &quot;nbit_adder:s3|s[11]&quot;  Error (12015):</p>
  <p>Net is fed by &quot;nbit_adder:s4|s[11]&quot;</p>
  </blockquote>
  <p>And more than like this. what should I do ?</p>