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: If Sarah writes a letter, then William is a nurse. If William were a nurse, then Sarah is a taxi driver. Sarah is writing a letter. If Susan sleeps, then Mary is not drinking water. If Sarah were a taxi driver, then Mary is drinking water.
William is a nurse can be inferred via the modus ponens rule. If Sarah writes a letter, then Sarah is a taxi driver can be inferred via the transitivity rule. If William were a nurse, then Mary is drinking water can be inferred via the transitivity rule.