message: Add virtual destructor.
diff: @@ -114,6 +114,8 @@ public:
  FaceRestriction(int h, int w): height(h), width(w) { }
  + virtual ~FaceRestriction() { }
  +
  /** Extract the face degrees of freedom from @a x. */
  virtual void Mult(const Vector &x, Vector &y) const = 0;
  