code: function() {
                  var e, expressions = [];
  
                  e = $(this.expression);
                  while (e) {
                      expressions.push(e);
                      if (! $(',')) { break; }
                      e = $(this.expression);
                  }
  
                  if (expressions.length > 1) {
                      return new tree.Value(expressions.map(function(e) {
                          return e.value[0];
                      }));
                  } else if (expressions.length === 1) {
                      return new tree.Value(expressions);
                  }
              }
docs: A Value is a comma-delimited list of Expressions In a Rule, a Value represents everything after the `:`, and before the `;`.