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 y_1, y_1 will write a letter if and only if y_1 is a sailor. There is at least one y_1 for which y_1 is not a taxi driver. For all y_1, if y_1 writes a letter, then y_1 is a taxi driver.
There is at least one y_1 for which y_1 will not write a letter can be inferred via the existential modus tollens rule. For all y_1, if y_1 writes a letter, then y_1 is a sailor can be inferred via the universal biconditional elimination rule. For all y_1, if y_1 were a sailor, then y_1 is writing a letter can be inferred via the universal biconditional elimination rule.