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: John is reading a book or Karen is sad. If John reads a book, then Sarah is playing squash. If Sarah plays squash, then Karen is sad. If Karen were sad, then Sarah is playing squash.
If John reads a book, then Karen is sad can be inferred via the transitivity rule. Sarah is playing squash can be inferred via the disjunction elimination rule. Sarah is playing squash if and only if Karen is sad can be inferred via the biconditional introduction rule. Karen is sad if and only if Sarah is playing squash can be inferred via the biconditional introduction rule.