title: AMPL loops and omitting variables
body: <p>Have started using ampl recently and have run into a hiccup regarding iteration  and indexing.</p>
  
  <p>I have the following model script:</p>
  
  <pre><code>set NUTR;
  set FOOD;
  
  param Cost {FOOD} &gt; 0; #Costs pr. gramm
  
  param f_min {FOOD} &gt;=0;
  param f_max {j in FOOD} &gt;= f_min[j];
  
  param n_min {NUTR} &gt;=0;
  param n_max {i in NUTR} &gt;= n_min[i];
  
  param amount {NUTR,FOOD} &gt;= 0; #amount nutrients pr. gramm
  
  var Purchase {j in FOOD} &gt;= f_min[j], &lt;= f_max[j];
  
  minimize Total_cost: sum{j in FOOD} Cost[j] * Purchase[j];
  s.t. Intake {i in NUTR}: n_min[i] &lt;= sum {j in FOOD} amount[i,j] *  
  Purchase[j] &lt;= n_max[i]; 
  
  data;
  
  set NUTR := PRT CRB FAT KCAL;
  set FOOD := CD VL HM SLM TN MCR EGG BN PS FSL PEN RC ORG PT ON AVC CLF CBG ALM TMT OLV MLK ;
  </code></pre>
  
  <p>The model runs fine with the dataset, but my question is:
  Is it possible to create a loop giving an optimal solution, omitting one product from the set FOOD at a time?</p>