You have had enough with all the UML tools on the market as none do exactly what you want them to. So you've decided to design your own. However when designing it you realize that certain parts will be really complicated for example you have a Diagram object that is made up of lots of other objects. This diagram object can be used for a variety of different diagrams including class and sequence diagrams. When you create it you only want to specify its type and content. What pattern should you use?