code: def _remove_dep(self, dep):
          
          refcount = self._deps[dep]
          if refcount == 1:
              self._deps.pop(dep)
              dep._dependents.pop(self)
          else:
              self._deps[dep] -= 1
docs: Decrement the reference count for *dep*. If the reference count 
          reaches 0, then the dependency is removed and its *changed* event is
          disconnected.