author(sevenlangs, tate).
author(rubyonrails, tate).
author(rubyonrails, hibbs).
author(sevenmorelangs, tate).
author(sevenmorelangs, dees).
author(sevenmorelangs, daoud).
author(sevenmorelangs, moffitt).
author(effectivetesting, dees).
author(effectivetesting, marston).
author(sevenwebframeworks, daoud).
author(sevenwebframeworks, moffitt).

coauthor(X, Y) :- \+(X, Y), author(Z, X), author(Z, Y).