Several recent knowledge-representation schemes have used virtual copies for storage efficiency. Virtual copes are confusing. In the course of trying to understand, implement, and use Jon Doyle's SDL virtual copy mechanism, we encountered difficulties that led us to define an extension of virtual copies we call virtual inclusion. Virtual inclusion has interesting similarities to the environment structures maintained by a program in a block-structured language. It eliminates the clumsy typed part mechanism of SDL, and handles properly a proposed test of sophisticated virtual copy schemes.