ABSTRACT
We now have four truth-functional constants: 'and', 'not', 'or' and 'if-then'. One more relation between statements is important when we apply logic to real life: it turns up everywhere, especially in laws and definitions. This is 'if and only if', abbreviated https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203061350/f5ce7580-7c0c-4392-aeab-0c1bd683fe20/content/fig236_B.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>
