You can, if you must, create a mock that derives from a concrete class. The problem is that the resulting class represents a mix of production and mocked behavior, a beast referred to as a partial mock.

I divorced that man because of you — It seemed I must do it, boundenly […]

Practitioners of Reality Mining soon discover the multitudes of analytic techniques suitable for their data sets.

On the Prelocutionary Act in Speech Events，https://web.archive.org/web/20160304103820/http://www.tronest.cn/Article.asp?id=1371565