LGTM. https://codereview.appspot.com/84680044/diff/1/scm/lily.scm File scm/lily.scm (left): https://codereview.appspot.com/84680044/diff/1/scm/lily.scm#oldcode753 scm/lily.scm:753: (diff (map (lambda (y) (apply - y)) (zip this last)))) This was impressive. `git grep zip scm` for a few more instances of (map ... (zip ...)) that could be cleaned up.
Replace a few more cases of (map ... (zip ...))