What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Richard is drinking water if and only if Mary is writing a letter. Mary is writing a letter if and only if Richard is drinking water.
If Richard drinks water, then Mary is writing a letter. If Mary writes a letter, then Richard is drinking water. If Mary writes a letter, then Richard is drinking water. If Richard drinks water, then Mary is writing a letter.