title: why AMPL can't solve the optimization failure
body: <p>I have a problem that I don't know how to solve. In my AMPL model, <code>1B12</code> is an item that belongs to a Set.</p>
  
  <p>And when I try to solve the problem by AMPL + cplex, the AMPL command show</p>
  
  <pre><code>presolve, constraint production['1B12']:
  no variables, but lower bound = 12792, upper = 12792
  </code></pre>
  
  <p>And the AMPL doesn't give me the optimal value I want. I am new to AMPL and can't know what the sentence meaning, especially the sentence</p>
  
  <pre><code>no variables, but lower bound = 12792, upper = 12792
  </code></pre>
  
  <p>And in fact, I checked that <code>1B12</code> is in my set when I load the data, thus I am confused why AMPL says <code>no variables</code></p>
  
  <p>I hope someone can help me solve this problem. Thanks! </p>