What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? If Mary writes a letter, then it is sunny. If it is sunny, then Mary is writing a letter. Mary is writing a letter if and only if Patricia is a musician.
Mary is writing a letter if and only if it is sunny. It is sunny if and only if Mary is writing a letter. If Mary writes a letter, then Patricia is a musician. If Patricia were a musician, then Mary is writing a letter.