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 is a soldier if and only if x_9 will write a letter. For all x_9, if x_9 were a soldier, then x_9 is drinking water. For all x_9, x_9 will write a letter or x_9 is a sailor. For all x_9, x_9 will not drink water.
For all x_9, x_9 is not a soldier can be inferred via the universal modus tollens rule. There is at least one x_9 for which if x_9 were a soldier, 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 if x_9 writes a letter, then x_9 is a soldier can be inferred via the existential biconditional elimination rule.