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 x_13, if x_13 plays tennis, then x_13 is writing a letter. For all x_13, if x_13 writes a letter, then x_13 is playing tennis. Susan is not a soldier. Susan is a soldier if and only if James is writing a letter.
If Susan were a soldier, then James is writing a letter can be inferred via the biconditional elimination rule. If James writes a letter, then Susan is a soldier can be inferred via the biconditional elimination rule. James is not writing a letter can be inferred via the biconditional elimination rule. For all x_13, x_13 will play tennis if and only if x_13 will write a letter can be inferred via the universal biconditional introduction rule. For all x_13, x_13 will write a letter if and only if x_13 will play tennis can be inferred via the universal biconditional introduction rule.