code: public Meta addTag(String theSystem, String theCode, String theDisplay) {
       addTag().setSystem(theSystem).setCode(theCode).setDisplay(theDisplay);
       return this;
      }
docs: Convenience method which adds a tag
  
  @param theSystem The code system
  @param theCode The code
  @param theDisplay The display name
  @return Returns a reference to <code>this</code> for easy chaining