ref: 61395a5141e590f4fcf0964c4a6a3d2ef55d1ed9
parent: f6a727649589c6fafff4d6b18a27e95726d85892
author: Simon Tatham <anakin@pobox.com>
date: Wed Apr 4 15:12:17 EDT 2007
Ensure the shuffling process never produces an already-solved grid. [originally from svn r7446]