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: It is not overcast. If it is overcast, then Mary is writing a letter. If Mary writes a letter, then it is overcast. If it is overcast, then Mary is writing a letter.
Mary is not writing a letter can be inferred via the modus tollens rule. It is overcast if and only if Mary is writing a letter can be inferred via the biconditional introduction rule. Mary is writing a letter if and only if it is overcast can be inferred via the biconditional introduction rule. Mary is writing a letter if and only if it is overcast can be inferred via the biconditional introduction rule. It is overcast if and only if Mary is writing a letter can be inferred via the biconditional introduction rule.