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 z_1, z_1 is a taxi driver if and only if z_1 is a scientist. For all z_1, if z_1 were a taxi driver, then z_1 is writing a letter. For all z_1, z_1 will write a letter if and only if z_1 is a scientist.
For all z_1, if z_1 were a taxi driver, then z_1 is a scientist can be inferred via the universal biconditional elimination rule. For all z_1, if z_1 writes a letter, then z_1 is a scientist can be inferred via the universal biconditional elimination rule. For all z_1, if z_1 were a scientist, then z_1 is a taxi driver can be inferred via the universal biconditional elimination rule. For all z_1, if z_1 were a scientist, then z_1 is writing a letter can be inferred via the universal biconditional elimination rule.