New usage of must_change?

[Sameer Siruguri]

I couldn't figure out how must_change should now be used.

http://blowmage.com/minitest-rails/Minitest/Rails/Expectations.html#method-i-must_change

The new expectation needs to be passed a block, per issue #181.
https://github.com/blowmage/minitest-rails/issues/181

But I don't see any documentation for what that block should do.

Thanks.