What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Name the inference rule being used: For all y, y will not read a book. There is at least one y for which if y writes a letter, then y is a sailor. For all y, if y writes a letter, then y is reading a book. For all y, if y were a sailor, then y is writing a letter.
For all y, y will not write a letter can be inferred via the universal modus tollens rule. For all y, if y were a sailor, then y is reading a book can be inferred via the universal transitivity rule. There is at least one y for which y will write a letter if and only if y is a sailor can be inferred via the existential biconditional introduction rule. There is at least one y for which y is a sailor if and only if y will write a letter can be inferred via the existential biconditional introduction rule.