Sets the value of an attribute of an <xml-element>. This function does not follow the normal -setter syntax. If attribute is not present in element, it is added.
sealed
| value | An instance of <string>. The new value of the attribute. |
| element | An instance of <xml-element>. |
| attribute | An instance of <string>. The name of the attribute. |
| value | An instance of <string>. The value argument. |