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 it snows, then Mary is writing a letter. If it is sunny, then it is snowing. If Mary writes a letter, then Thomas is sad. If Jessica were a server, then Thomas is sad. Jessica is a server or it is sunny.
If it snows, then Thomas is sad can be inferred via the transitivity rule. If it is sunny, then Mary is writing a letter can be inferred via the transitivity rule.