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: There is at least one x_9 for which x_9 will write a letter if and only if x_9 will read a book. For all x_9, x_9 will read a book if and only if x_9 will take a plane. For all x_9, x_9 will not write a letter.
For all x_9, if x_9 reads a book, then x_9 is taking a plane can be inferred via the universal biconditional elimination rule. There is at least one x_9 for which if x_9 writes a letter, then x_9 is reading a book can be inferred via the existential biconditional elimination rule. For all x_9, if x_9 takes a plane, then x_9 is reading a book can be inferred via the universal biconditional elimination rule. There is at least one x_9 for which if x_9 reads a book, then x_9 is writing a letter can be inferred via the existential biconditional elimination rule. There is at least one x_9 for which x_9 will not read a book can be inferred via the existential biconditional elimination rule.