ABSTRACT
At times we want to say explicitly that a set is NOT empty. We do this by drawing a stroke in the circle which represents that set. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203061350/f5ce7580-7c0c-4392-aeab-0c1bd683fe20/content/fig299_B.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>
This says that there is at least one unicorn.
