code: function (inValue, getLookup) {
                      var s = parserInput.currentChar(), important = false, lookups,
                          index = parserInput.i, elements, args, hasParens;
  
                      if (s !== '.' && s !== '#') { return; }
  
                      parserInput.save(); // stop us absorbing part of an invalid selector
  
                      elements = this.elements();
  
                      if (elements) {
                          if (parserInput.$char('(')) {
                              args = this.args(true).args;
                              expectChar(')');
                              hasParens = true;
                          }
  
                          if (getLookup !== false) {
                              lookups = this.ruleLookups();
                          }
                          if (getLookup === true && !lookups) {
                              parserInput.restore();
                              return;
                          }
  
                          if (inValue && !lookups && !hasParens) {
                              // This isn't a valid in-value mixin call
                              parserInput.restore();
                              return;
                          }
  
                          if (!inValue && parsers.important()) {
                              important = true;
                          }
  
                          if (inValue || parsers.end()) {
                              parserInput.forget();
                              var mixin = new(tree.mixin.Call)(elements, args, index, fileInfo, !lookups && important);
                              if (lookups) {
                                  return new tree.NamespaceValue(mixin, lookups, important);
                              }
                              else {
                                  return mixin;
                              }
                          }
                      }
  
                      parserInput.restore();
                  }
docs: A Mixin call, with an optional argument list  #mixins > .square(#fff); #mixins.square(#fff); .rounded(4px, black); .button;  We can lookup / return a value using the lookup syntax:  color: #mixin.square(#fff)[@color];  The `while` loop is there because mixins can be namespaced, but we only support the child and descendant selector for now.