The computer has brought a similar fluidity to many other media: artistic drawings, building plans, mechanical drawings, musical compositions, photographs, video sequences, slide presentations, multimedia works, and even to spreadsheets. In each case, the manual method of production required recopying the bulky unchanged parts in order to see changes in context. Now we enjoy for each medium the same benefits that time-sharing brought to software creation--the ability to revise and to assess instantly the effect without losing one's train of thought.