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_13, x_13 is a sailor if and only if x_13 is poor. For all x_13, x_13 is sad or x_13 is poor. There is at least one x_13 for which if x_13 were sad, then x_13 is a sailor. For all x_13, if x_13 were a sailor, then x_13 is a taxi driver.
There is at least one x_13 for which if x_13 were sad, then x_13 is a taxi driver can be inferred via the existential transitivity rule. For all x_13, if x_13 were a sailor, then x_13 is poor can be inferred via the universal biconditional elimination rule. For all x_13, if x_13 were poor, then x_13 is a sailor can be inferred via the universal biconditional elimination rule.