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: For all x_12, if x_12 were a sailor, then x_12 is a carpenter. David is running. Charles is not a sailor. For all x_12, if x_12 were a carpenter, then x_12 is a sailor.
David is running and Charles is not a sailor can be inferred via the conjunction rule. Charles is not a sailor and David is running can be inferred via the conjunction rule. For all x_12, x_12 is a sailor if and only if x_12 is a carpenter can be inferred via the universal biconditional introduction rule. For all x_12, x_12 is a carpenter if and only if x_12 is a sailor can be inferred via the universal biconditional introduction rule.