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: There is at least one x_13 for which x_13 will take a plane or x_13 is a taxi driver. For all x_13, x_13 will write a letter if and only if x_13 is a taxi driver. For all x_13, if x_13 takes a plane, then x_13 is writing a letter. For all x_13, x_13 will work.
For all x_13, if x_13 writes a letter, then x_13 is a taxi driver can be inferred via the universal biconditional elimination rule. For all x_13, if x_13 were a taxi driver, then x_13 is writing a letter can be inferred via the universal biconditional elimination rule.