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_11 for which x_11 will read a book. For all x_11, if x_11 writes a letter, then x_11 is not reading a book. For all x_11, x_11 will play a game if and only if x_11 will write a letter.
There is at least one x_11 for which x_11 will not write a letter can be inferred via the existential modus tollens rule. For all x_11, if x_11 plays a game, then x_11 is writing a letter can be inferred via the universal biconditional elimination rule. For all x_11, if x_11 writes a letter, then x_11 is playing a game can be inferred via the universal biconditional elimination rule.