The necessary and sufficient condition for the establishment of A is that B is established.

This sentence can be divided into two parts:

The necessary condition for the establishment of 1 and a is that b is established.

2. The sufficient condition for the establishment of A is that B is established.

For the case 1, the literal interpretation means that B is necessary, and there is no A without B. By using the equivalence principle of negative proposition, B is deduced from A..

Example 2, naturally, B deduces A, and literally, B is sufficient and sufficient to deduce A..