Abstract system {\em -- exemplary interface}

\zline{\fbox{\fbox{library}}}
  p = new person();
  b = new book();
  p = b->borrower;
  s = p->books;
  tf = b->inlibrary();
  b->borrow(p);
  p->allocate(b);
  p->deallocate(b);
  b->_return(p);
  

slide: The library system