Deliberately placing something in foreground is called posting, and what is so placed is called a post. A key point is that anything in foreground is automatically taken as necessary. This is not difficult to understand. After all, foregrounding something is saying it’s worth the attention of others, and all of us are busy, so why bother us about something that isn’t necessary? (So, the implied necessity of things in foreground can be taken as an theme within Social Psychology.) Therefore a post stating that something is sufficient also means, unless explicitly so stating to the contrary, that it is necessary. That is why in public discourse one seldom sees the expression ‘if, and only if,’: the ‘and only if’ part is universally presumed. Textbooks are very much part of the public discourse, and that is why, even in mathematics, definitions are of the minimal form ‘if’, rather than of the full form ‘if, and only if’, for example, ‘x is a prime if x is an integer > 1 such that…”. Another example is the sign posted for day workers informing them that they can cash their check at a certain convenience store. If you don’t understand the implied necessity and try to cash your check elsewhere, even at the bank, you will find that you wasted your time.
The test is available here.
If you think that the nominative case must follow a form of the verb ‘to be’, tell Vikki Carr.
(reference to her famous song ‘It Must Be Him’)