public abstract vVariable Swap(vDescriptor x);
public vVariable Swap(vDescriptor v) { vValue a = this.Deref(); vValue b = v.Deref(); vVariable retval; if ((retval = this.Assign(b)) == null || v.Assign(a) == null) { return null;  } return retval; }
public abstract vVariable Swap(vDescriptor x);		// v :=: x