code: function(key, value) {
              console.log('add attribute', key, value);
              if(! value) {
                  return;
              }
              if(VCard.multivaluedKeys[key]) {
                  if(this[key]) {
                      console.log('multivalued push');
                      this[key].push(value)
                  } else {
                      console.log('multivalued set');
                      this.setAttribute(key, [value]);
                  }
              } else {
                  this.setAttribute(key, value);
              }
          }
docs: Set the given attribute to the given value. If the given attribute's key has cardinality > 1, instead of overwriting the current value, an additional value is appended.