And they've been such since the 70s: http://homes.cs.washington.edu/~levy/capabook/ http://homes.cs.washington.edu/~levy/capabook/ (This book is well worth rea

And they've been such since the 70s: http://homes.cs.washington.edu/~levy/capabook/ http://homes.cs.washington.edu/~levy/capabook/ (This book is well worth reading...)